I would agree with The Great Mighty Doom above on the Special Sweeper thing. I've tried using a Venusaur myself, and for me, I found that Venusaur just lacked the coverage with Giga Drain/Sludge Bomb/HP Fire. It hit hard, but the coverage afforded by Victreebel's Grass/Fire/Ice hits Dragons and the new Therians hard. (understandably, a SE HP Ice has only 5 more BP than a neutral Sludge Bomb, but against Dragonite/Salamence, the power increase is notable). I haven't actually tested Victreebel myself, but at least in theory, it seems pretty nice. The lower speed does mean that you'll need to be more careful about things that have already set up a speed boost though.
Just my 2 cents.
hope I helped!