Right now, I'm not seeing a difference between Vileplume and Bellossom. I would give Vileplume Poison Drain, as it gives it a handy immunity, making it one of the best partners for a fairy teammate. As far as Bellossom, I would recommend Leaf Guard.
In order to make them different, I would gear each one a little differently. Maybe making Vileplume a little more offensively based by lowering its special defense to 105, and then making Bellossom more defensive by lowering its special attack to 105, or something.