You should definitely pledge allegence to one now....if not, you are just losing the rep. It works different after 2.0. If you don't pledge, you only get vanguard rep. If you do pledge, you get the exact same amount of vanguard rep, plus faction rep. Before you just got one or the other. No reason not to now....Unless you just are opposed to getting more packages / rewards.....plus faction quests that are supposed to give exotic class items.

So i still get vanguard rep?

Yes, it's been changed in 2.0. You still get the same amount of vanguard rep, plus faction rep.......and you can wear whatever class item you like....only the faction pledge makes any difference, not the item you wear.