VSV Valve and Cannister

I took my 2000 Sienna in to be looked at and was told the VSV valve and cannister needed to be replaced. Someone told me the cannister should be replaced but the valves should be OK. The service station said both should be replaced because one is causing the other to fail. I don't know who to believe so I thought I would check here. Does anyone know more about this than me that could help. The total cost difference is $500 vs. $250 so I want to make sure I'm doing the right thing.