I have never heard anyone use the word "vata" (I have heard Mexican-Americans use the word "vato", but only to refer to a man).
As far as your roommate, could you ask her?
I spend a lot of time in Mexico and have Mexico friends. With people that I just meet, I am generally formal. I have friends that call me "Guey" and "Cabron"... and I feel comfortable using those words (even so, I don't use them very often... I am the foreigner).
In general political correctness is much less of a thing in Mexico. Americans worry about everything, since it might take offense. In other countries it is not so much.
There are women I know who are called "gorda (fat)" as an affectionate nickname. I know someone who is called "Negro" because of his dark skin, and someone else who is called "China" (she is not Asian at all) because of her appearance. These things would offend many Americans. Mexicans.... not so much.
That being said, as a foreigner you should be extra careful. There are some things that might offend someone... you just have to learn them. It also varies from State to State, in the North of Mexico it is far more common to hear "cabron" used affectionately. In some places I think it would be considered offensive.
As far as your roommate, if you have a level of friendship you can ask her in a respectful way.