Im vilified with NCR, and i can still walk into their towns, trade, etc. Why? For example when i became vilified with Powder Gangers they always attacked me.

Is there any other factions expect NCR which dont attack you at vilified?

(i dont wear faction armor)

It's a back up security plan in my guess.
As a lot of the game is set in NCR held lands, having them automatically hostile for a new player ( which can happen ) would be a bad idea.
So you're left with just geting hunted down by rangers untill later on after Vegas.