I currently have Vanilla planifolia growing in two tanks, but it grows very slow. I wouldn't count on it to really make much of a difference as far as water quality. you have to be sure to keep the stem out of the water, it will rot pretty quickly.
if you can get some lizard tail plant, mine in the tank are doing well, and is growing roots from multiple spots on the stalk. the ones I planted in my emperor 400 don't seem to like so much flow, but it could just be that they don't get much light either.
I just brought home a Chlorophytum comosum cutting that I hope to grow out of my overflow. pretty fast grower with lots of hanging plantlets, I fully expect this baby to suck down some nitrates.