On the general topic of their relative ninja-ness: Vileplume definitely looks like like it'd be bad at stealth and mobility, particularly compared to Bellossom, but looking at their actual stats as seen on the research page, the two have the same AGI (a piddly 5). Vileplume has Moonlight Walker (doubles AGI at night) and Bellossom has Chlorophyll (doubles AGI in harsh sunlight) and Dancer (increases basic accuracy and evasion while dancing).
Vileplume's AGI boost is theoretically more accessible, since it just requires nightfall. Gloom and Vileplume can also use Stench, which causes nearby units to lose 2/4 AGI-- great while solo, but it doesn't seem to exclude allies. Vileplume has better control over the stench, though, so it can probably cancel that ability when it needs to play nice. That said, it's got a terrible natural movepool (
Solar Beam for a nocturnal pokemon seriously), particularly if start moves aren't counted; really hope Gloom's moves carry over there like they can in the games. Otherwise, Vileplume's only Poison STAB is Acid and its Grass options range from "Let me suck a little bit of your life energy? Pretty please?" to "MY PETALS WILL BLOT OUT THE SUN", so we're gonna have trouble not accidentally killing things in the midrange. Heck, even with Gloom's moves Vileplume's only Poison STAB is Acid. <.< Toxic is pretty nice, though.
Bellossom... oh boy. I don't know how much of this is going to transfer over, but actually giving it an in-depth look, keeping in mind the fact that teaching new moves isn't gonna be nearly as simple as buying a TM (if it's even possible), Bellossom may be much better for us than even I thought? Its AGI boost is harder to get if we don't have a natural Sunny Day user, but if start moves count, Bellossom
is a natural Sunny Day user.
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It's got a
really nice natural movepool, in fact. Its start moves
include Leaf Storm, which already makes it a glass cannon, as well as Leaf Blade (which is a high crit ratio move; very appropriate for a ninja) and Sweet Scent. Upon evolution, it gets the guaranteed-hit move Magical Leaf.
Finally-- and this bears its own paragraph-- it has
Quiver Dance, a status-increasing move that's pretty damn incredible in the games; translated here it's probably not quite as good, but it'd at least raise both AGI and INT and synergize with Bellossom's Dance ability. The only obvious omissions are Solar Beam (
why does Vileplume get Solar Beam and not Bellossom why), Sleep Powder, and Stun Spore, though we can hope those last two translate over from Gloom.
Bellossom's still no powerhouse given its stats, but if we capture an unusually strong individual and train it well (especially some elemental resistance training) it could maybe hold its own even once we've started filling in our party?