I am still bit wondering about reputation part of NV but it seems that if you kill member of a faction wearing that faction uniform, you are immediately vilified. Do you have same experience?

I dunno, disguises aint working for me

I stole from a faction THEN wore there uniform but it didnt do anything, they still attacked

It worked for me just fine. Guard dogs and certain NPCs can see through your disguise however. Those are mostly named NPCs so stay clear of them. Also as far as I did not use guns, only those who discovered me turned hostile, rest of them just ignored me.

Also if you change your dress in plain view of them, disguise will not work. It takes some time until they "forget about you". I accidentally wore powder ganger uniform in to the quarry camp and got hailed by bullets. It was of no help that I changed dress on realising my mistake. I had to leave area and came back later.

I had Caesar Assasins running up to me to attack, I just threw on my trusty Legionnaire uniform and they stopped dead!