Does temperature affect vampires? Do they feel it change? Do they sweat when it's hot? Do they tremble when it's cold?

I've wondered this, too.  In The Initiative, the commandos say Spike is room temperature, though their heat instruments read him at 60 something, which is hardly room temp.  Maybe they were just being quippy about it.  In Guise Will be Guise, Angel says: "I don't have a body temperature so..." when the swami asks him about wearing leather in 80 degree heat.

Either vampires ARE somewhat room temperature, and are whatever temperature they are around.  Or they are always cold.  If they changed temperature, I would think that temperature changes would affect them.  If they were always cold, I would think that temperature changes wouldn't bother them (unless perhaps it was extreme).  I tend to think they're always cold, unless they've just fed.

The actors sometimes sweated, but that doesn't mean that vampires necessarily sweat.  (Sweating also gets into that whole 'how can they have sex without circulation' and other body functioning questions.)  I think Angel sweated when he was poisioned in Graduation, but I'm not sure.

I remember in Pangs, Spike was covered with a blanket at night because of the cold, which made me a little confused.

I think the physical abilities and characteristics of vampires are things that were blown off in canon -  like Angel's "I have no breath" comment when Buffy needed CPR, which is patently ridiculous because without it his vocal chords wouldn't be allowing him to speak.  The writers were pretty inconsistent (wow, there's a surprise...) with most of those things, so there are  no hard and fast rules that I know of. They contradicted themselves so often that there aren't very many things that can really be said to be "canon" for the BtVS vampires.

There have been a lot of meta posts through the years (some may be here, on the forum) about the physical characteristics of vampires; what they can and can't do; explanations for how they can have some bodily functions and not others, etc. They can be really interesting, but ultimately there is no final answer because it's all fiction anyway. Each writer must decide upon his/her own mythology and physiology and try to stick with  - at least in the course of any one story. 

My opinion is - these are imaginary creatures and each writer has twisted the mythology to suit his/her own ends. Some of the basics - stake through the heart, cut off the head, sunlight is a no-no, etc are pretty well-established going back to Stoker or wherever he got his ideas.  However, much of vampire lore is situational and depends upon the needs of the writer.  So, St. Germaine has to keep dirt in his shoes, but other vampires don't care a fig for their native soil.  Certainly Sunnydale vampires have no issues with crossing water, needing  to sleep in coffins, etc.  Nor do they (except for when Dracula showed up) have any sort of human helpers.

I think, as long as you remain consistent throughout your own story (or throughout all your stories, if you want to maintain whatever mythology you've settled on as correct for "your" vampires), then you can write them any way you like or need. Bearing in mind the things (see above) that are so well-established that contradicting them would be tantamount to inventing a whole new creature. lol

Was Spike really using that blanket cause he was cold, or cause he was trying to hide that shockingly white hair cause he was in hiding?  Also, he needed the blanket to get around in the daytime.

Also, here's a clip from "Guise Will Be Guise" :6
Magev:  "Oh. So why all the layers, all the black?  You know it's been about 80 degrees in the shade lately."
Angel:  "No reason.  I-I don't have a body temperature so..."
Magev:  "So it's for the look."
Angel:  "No. - It's just this way I don't have to worry about matching. - I don't have a reflection so..." 
 

I realise that someone else posted a shorter clip of this convo, but I do think an important part, particularly for Spike and Angel, is image (in regards to wearing those leather coats in the summertime.  Vampires don't have a heartbeat and don't produce their own blood, so they can't be 98.6.  I think they're a bit like amphibians.  I'm not a major in biology or what have you, but I'm guessing the answer is one of these:  Whereas an organism that thermoregulates is one that keeps its core body temperature within certain limits, a thermoconformer is subject to changes in body temperature according to changes in the temperature outside of its body

I'm thinking vamps thermoregulate more than thermoconform though.  I say that because when it is hot, then naturally, they'd become hot too.  But if they were hot, they'd stop wearing those hot ass leather coats, which they don't.  Anyway, that's my take on it :P

Not necessarily so - if they FELT hot, in an uncomfortable way, then they would take their coats off. If their bodies changed temperature and it didn't bother them (if indeed they were even aware of it) then the fact that it was changing temperature wouldn't matter at all. I realise that they must be at least a little aware of temperature (fire = pain etc), but perhaps not bothered by those within the ambient temperature ranges. As they are dead, there is no reason for their body to respond to temperature changes with discomfort (no enzymes to denature etc).

Long ramble over, my point is that they could be thermoreguators or thermoconformers...except that if they were thermoregulators they should should thermoregulating behaviour (shivering, sweating) which they don't.

You know... I just noticed that Angel seems to say that he doesn't have something when what he means is, what he has is not normal (see also "I have no breath" ). "I don't have a body temperature" - um, of course you do. Everything has a temperature. What you mean is, you don't have a normal, regulated body temperature. A vampire's body temperature changes based on the temperature of the surrounding air (which, to go along with Scarlet's biological terms, makes them thermoconformers), which is why they don't feel hot in their black and leather (or, presumably, cold in the winter).

Two things I'd like to give my take on:

1 - The blanket it Pangs: I thought Spike seemed cold as well, but I thought it was moreso a reaction to the lack of blood. By that point, he hadn't eaten for a few days, and he was probably experiencing symptoms of both withdrawal and hunger, which could cause chills and involuntary shaking. Combine that with general weakness from hunger and the fact that he probably hadn't shivered in decades, and his body's gut reaction would be to tell him that "You're shaking, so you're cold." Hence the blanket at night.

2 - Angel and the "I have no breath": I think this was just a really dumb mistake on Angel's part. If you think about it, it's something that, in his poetic mind (because he does try to be overdramatic in his brooding), he doesn't have any breath in the life-giving sense. And technically, he doesn't, the breath, like the blood running through his body, has no chemical reactions going on. So he was -- without thinking it through in the heat of the moment -- stating very matter of factly, I'm not alive. I couldn't give a blood transfusion or anything like that. I don't breathe like a human does, so I certainly can't give CPR.

Ironically, though, it would have been *better* for Angel to give Buffy CPR, because, since he only uses air to talk and not absorb oxygen into his blood, he would still channel oxygen into her, as opposed to predominantly carbon dioxide like Xander does.

My opinion is - these are imaginary creatures and each writer has twisted the mythology to suit his/her own ends. Some of the basics - stake through the heart, cut off the head, sunlight is a no-no, etc are pretty well-established going back to Stoker or wherever he got his ideas. However, much of vampire lore is situational and depends upon the needs of the writer. So, St. Germaine has to keep dirt in his shoes, but other vampires don't care a fig for their native soil. Certainly Sunnydale vampires have no issues with crossing water, needing to sleep in coffins, etc. Nor do they (except for when Dracula showed up) have any sort of human helpers.

I think this is pretty much how the series worked - the vamps characteristics were used to suit the weekly stories with particular traits to distinguish the character for the series - like Angel only drinking blood and not eating food and Spike eating human food.  When Angel is poisoned his bodies responds with human symptoms - Spike is shown using a blanket in "Never Leave Me" and he wears jackets and sweaters which would normally imply response to temperature.  When you see him encounter Nikki and get his Slayer duster trophy, she wears that heave leather coat and Spike is in a sleeveless t-shirt.  Whatever the story needs to work seems to be the working formula.