I am currently playing a level 28 mage vampire with vampirism at 100%, and man, is it awesome. It pretty gives +20 in Intelligence, Destruction, Speed, Strength, and a bunch of other stuff that's very useful, while the downsides are that I can't go outside during the day, which is minor problem since I can just wait, and the 50% weakness to fire, which is surprinsingly not that big of a deal. I tried feeding to see how my mage could handle himself, and it svcks. I have a bunch of enchants that overall give me + 150 magicka and 20 Strength (I have a 510 weight limit ^^). With that, I made a custom spell which does 99 damage on touch and has a Soultrap effect, so I can fill Azura's Star all the time. Basically, I feel powerful, which is a god change from Skyrim's frustrating magic system.

Does anybody here also play as a vampire for the awesome benefits?

Also, quick question: Since my Vampirism fortifies my Intelligence by 20, does it mean that I'll have to feed before I can level up Intelligence again (I've reached the limit) ?

I don't think you need to feed. When you go to the level up menu, it should so your base intelligence. So in this case, I'm assuming the +20 to intelligence makes it appear as a green 100 to you, but when you level up, it'll show it around 80.

I wanted to play as a vampire as part of my role play but it came much too fast for me so I cured it. I made a separate save in case I decide otherwise want to play as a vampire at some point but I wanted to be much stronger and I still had a lot of stuff to do that would have preferred sunshine to it. At least if it's on another character I can then try it on a modded version (I have a list of mods I want to try)

I didn't consider it as a mage though, that would be interesting for the power and strength.

As for your intelligence level, hagibis answered that pretty well. Though when I was a vampire it added the strength bonus attribute at 25% (when I was first turned), but when I went to level up it sat at 100 and I couldn't level it any more, but when I cured myself and then leveled it (different save) it sat at 95 and I could add to it. I didn't check if it was green, but so long as the attribute is green, you should be able to level it no problem.

I just checked, and it is not the case. I guess I'll have to feed

I love playing a vampire in Oblivion, the power it feels good, and what is so nice is you can be pretty much any class as a vampire and it enhances your abilities. Pretty much with every character I end up a vampire mainly on purpose, but sometimes it is an accident and I am stuck like that since I have the GOTY edition of Oblivion.