It can stall with Sleep Powder+Leech Seed combos, but it's SA is its greatest asset, right?
Its a mix of both. It puts the enemy to sleep, leech seed, then uses growth and murders with energy ball.
Edit:I only would use venasaur in sun with its dw ability with this set.
Depends on the weather. If you have Sun up and you have its DW ability its definitely should be a sweeper as it can pretty much outspeed everything. But without the sun and its DW ability its better off being a Tank.
Generally it is used as a sweeper, usually of the set-up variety.
In some cases it is used as a staller with the SubSeed strategy.
But it does not fall under the category of wall.