I'm assuming this is ingame?
There are advantages and disadvantages to both Vileplume and Bellossom. Both are equal in terms of HP, attack, defence, and speed. Also, both have equal 2x weaknesses to Flying, Fire, and Ice, and equal 1/2x resistances to Water and Electric. Both have the same special ability of Chlorophyll in the later generations.
Vileplume has the higher special attack. Also, Vileplume is part Grass type and part Poison type. Therefore, Vileplume resists Fighting 1/2x and Grass for an additional 1/2x, adding up to a total Grass resistance of 1/4x. However, Vileplume will gain a 2x weakness against Psychic type.
Bellossom has the higher special defence. Bellossom is a pure Grass type, and will therefore have a 1/2x Ground resistance (which Vileplume doesn't have because of its part Poison type). Also, Bellossom's Grass type resistance is only 1/2x. Bellossom doesn't have a weakness to Psychic type attacks due to only being a Grass type, but will have a 2x weakness against Poison and Bug type attacks.
Depending on the generation game you're in, their moves can be quite even or really close. In generation 3, both get petal dance, but only Bellossom can also get Solarbeam, which is the far more powerful move. In generation 4, Vileplume would seem to win, getting both petal dance and Solarbeam, while Bellossom only gets magical leaf (an average damage move that always hits) and leaf storm (a powerful move that weakens the user afterwards). In generation 5, Vileplume takes the cake as the moves are the same as in generation 4, but petal dance has been made even more powerful.
Final decision: Vileplume.
Vileplume has the better special attack, and can thus do more damage with its powerful Grass attacks. Also, Vileplume will get STAB (same type attack bonus) for two types of moves, thus allowing for a more diversified move set and greater versatility. Just to tell you this answer is so formal because I'm revising for an English Test.
Vileplume vs Bellossom? Vileplume for the win.
Hope I helped.