"is it because I am a Vampire now or not?"
Nope, vampirism doesnt influence any quest or its outcome. You can even do quest where you join vampire hunters to hunt down evil vampires while you are a vampire.

how is it saying that you are unable to loot the garb??
The most obvious reason you get this is cause you allready have the disguise in your inventory. It can also be that you allready have a disguise equipped (the dunmer garb or any other disguise).

It can also be that your quest bugged. In that case abandoning it (and removing all disguises equiped or not) and restarting the quest should help it.

marcoluco wrote: »

I lost the cultural garb, somebody know where i can find another, please?

You can try and abandon the quest and restart it which should let you get the disguise again. Be warned though this can have very strange consequences and sometimes just makes things completely worse.