Toyota Repair Problems? Ask a Mechanic for Answers ASAP
When the VSC light sets on this vehicle it indicates the presence of a trouble code in the computer. Did your mechanic scan the computer for trouble codes before changing the battery?
The VSC light can trigger for countless reasons so without a code to give you an idea where to look it will be difficult to know where to begin diagnosis.
That's tough to determine from just a VSC light. How many miles does your vehicle have?
I don't know that I would condemn the transmission at only 120k miles. The sienna typically has a fairly robust transmission. I think I would wait for the VSC and check engine light to set again and then scan for trouble codes. It could be something as simple as needing a tune up-it it hasn't already been done (usually due at 100k).
Did you need any additional help with your vehicle? Please don't forge to rate if I have been of some assistance.