In a Vector2, X and Y seem fairly obvious. Support Vector, the definition seems a bit different than how Dirk defined it (I kinda love that someone mentions the GDC slides of someone, and then that someone then replies to the thread). If you're looking for the vector orthogonal, that's not right.
Vector2 perpVector = new Vector2(-Y, X); (or the negative of that for the other direction, and I'd have to double check, but that first one is the 'left' vector, the negative is the 'right' vector, if you consider the existing vector to be 'forward')
Edited by ferrous, 01 April 2014 - 05:19 PM.