Victreebel isn't simple to use. It's slow and fragile and lacks a good typing. However, its attacking stats are through the roof. It has a hard time with a lot of standards, mostly Steels, but can nonetheless be a menace by hitting hard on switches. Still, not recommended for newer players. In UU, he can be used to make fun of Meganium, with its immunity to Toxic, 4x resistance to Grass moves, and a Swords Dance-boosted Sludge Bomb that will even shut down those with Reflect or Counter.
Victreebel can be a menace when used right. A Swords Danced Sludge Bomb OHKOes a lot of things. Ground and Rock Pokémon get promptly Giga Drained, particularly Swampert and Rhydon in standard and Quagsire and Golem in UU. Sleep Powder is to disable one counter. Save Swords Dancing for later in the game, when there's only slower stuff left that Victreebel KOs easily.
Hidden Power Fire catches Metagross and Skarmory on the switch and can deal damage to Gengar. Sludge Bomb is to make sure Snorlax and Regice aren't going to wall you easily. It helps against Blissey as well, though she will usually shrug it off with Softboiled. You won't want to cut your secondary attacking stat this time around, because without this Attack investment you won't do much to Snorlax and Regice at all. Rash is probably the best personality for this set, because with Mild, Dugtrio can kill you a lot easier.
Sunny Day Victreebel. Hidden Power Fire is for Metagross and Skarmory, while Sludge Bomb keeps Blissey and other Special walls at bay. Sleep Powder is helpful for stuff like Fire Pokémon and Salamence, but it's not totally necessary. Since you're going to try and just sweep your opponent, defensive investment isn't needed.
This set's main problem is that unless you use Sludge Bomb, Exeggutor outclasses it in every way in OU, so you should either use Sludge Bomb or use Exeggutor. In UU, however, Exeggutor is banned, so this problem no longer exists.
Encore can catch a lot of stuff off guard, such as Celebi's Recover, Salamence's Dragon Dance, Weezing's Haze, and Tyranitar's Substitute. Synthesis can work, but Victreebel is rather fragile and has no Ground resistance unlike the Recovering Celebi. Growth can be used to power up Special Attack, but Victreebel is bad enough at setting up as it is. Growth really doesn't increase your power that much, and Grass isn't a good attacking type at all. Stun Spore and Body Slam's paralysis chance help Victreebel's low Speed. Reflect can support the team. Hidden Power Ice is nice for catching Salamence and Flygon off-guard in standard, or Altaria and Gligar in UU.
208 Speed (128 EVs) is a good start for most if not all Victreebel's sets, especially in standard. It beats most Metagross, as well as max Speed Jolly Marowak. Suicune and Kingdra start at 206, so if they have only 4 Speed EVs or none at all you beat them as well. With your other EVs, you will want to max out Victreebel's main attacking stat. And then finally, when using Sludge Bomb on a Special set, put the leftover EVs in Attack. If you're not running Sludge Bomb like that, put what you have left into HP for some more survival, or Speed.
Checks and Counters
For standard: Weezing resists Sludge Bomb and Grass moves, can Will-O-Wisp Victreebel and take him down with Sludge Bomb or a Fire move. Haze will erase Swords Dances. Skarmory, Metagross, Jirachi and any other Steel are immune to Sludge Bomb and resists Grass. All three of them can hit Victreebel with STAB super effective moves, but Metagross will do fine with Meteor Mash. Hidden Power Fire is sour for them, though. Salamence and Gyarados take about 30% from Sludge Bomb after Intimidate and hit hard with Hidden Power Flying, and Salamence resists Grass 4x as well. Gengar and Dusclops resist Victreebel's STAB as well. They can both Will-O-Wisp, but Gengar can also use Ice Punch for a load of damage.
In Underused matches, Muk resists Sludge Bomb and Grass moves and can Sludge Bomb or use a Fire move on Victreebel's low defenses. Altaria resists Grass moves, but should watch out for these dangerous Sludge Bombs. He doesn't mind Victreebel's status employment though. Camerupt resists Sludge Bomb and OHKOes with his Fire moves, but a SolarBeam nearly OHKOes it. Gligar walls physical sets nicely with his good Defense and natural Sludge Bomb resistance, but Grass moves will hurt him. His Hidden Power Flying really hurts Victreebel though. Any Fire type is going to OHKO Victreebel, they should all beware of that nasty STAB Sludge Bomb, but if Victreebel uses Sunny Day they can really take advantage of that. Beware of Misdreavus as well, who can Thunder Wave or Hidden Power Ice Victreebel.
All of Victreebel's counters risk being Sleep Powdered, so try to absorb that with a Sleep Talker first, or put Sleep Talk on a counter.