In the final episodes of Angel's second season, the characters entered another dimension where things were very different.
Angel went under some very interesting changes. Most of his vampire weaknesses were gone, the sun no longer set him on fire, and he could see his reflection. However, when he tried to put on is vampire face, he turned into a demon and lost control over himself. The changes that affected Angel, I will assume, would affect all vampires. But there's really no way to be sure.
A few theories as to why this happened. First off, since this was another dimension, and had another sun, it's possible that if it's the physical properties of sunlight that causes a vampire to burn, that the physical properties of this sun are different, and so, don't burn vampires. Perhaps the light of the world is affected as a result and this is why Angel could see his reflection. Another possibility is that the reason a vampire has no reflection and burns in the sunlight is because these are powerful spells cast at the dawn of the age of humans and vampires. Since this is another dimension, these curses would not apply here. Either theory works, I tend to like the second one better.
This still doesn't explain why Angel turned into a beast, my theory on that is little more complex. I believe, that for whatever reason, Pylea effected Angel's supernatural physiology, and basically, divided his demon half and his human half.
Lets hold that thought for a minute and remember what a vampire is. The first vampire was created when an unknown species of demon mixed it's blood with a human's blood. This created a vampire, essentially a new species of demon that reproduces through mixing blood with a human, and on some sort of mystical level, this creates a new vampire. When a vampire is in it's natural human state, it is basically a human, little of the vampire powers transfer over. The only things that do are some enhanced senses (which are probably less powerful when in human form) and all the weaknesses that reveal the person to be a vampire (another reason why I believe that the weaknesses are the results of spells.) When a vampire puts on the vampire face, they have all their demon powers.
With that explained, lets get back to Pylea. When Angel was in his human form there, nothing was changed except for the lack of vampire weaknesses (see above.) But when he tried to put on his game face, he turned into a pure blooded demon. In my opinion, this is what the demon that created the vampire would look like. I refer to it as a Vantal, which is what the people of Pylea referred to Angel as. If you look at Angel in Pylea, when he's in his human form, he's much more human, and when he's in his vampire form, he's all demon. I think it's safe to assume that the splitting of his two halves is what happened to him while in this other dimension. It would be interesting to see what would have happened if Doyle had visited as well, as he's a mixed blood demon like Angel is.