The Vector is statistically the worst SMG in the game. If you aren't using it for nostalgic reasons or if you aren't going for diamond SMGs I wouldn't touch it. The MP7 outshines it in pretty much every way.
Either will work but I actually prefer the Vector. For some reason I can destroy people with it. The only reason that I would pick the PDW over the Vector would be if the enemy has a tendency to move in groups which would cause me to need the larger mag of the PDW.