Actually the same exact thing just happened to me today. I have a '03 GS300, 88k miles and my VSC lights came on.
I had the timing belts and spark plugs changed yesterday and on my way home from the dealer, the VSC and check engine lights came on. They've never come on before.
I took it straight back to the dealer and they claimed it was an O2 sensor. They changed the O2 sensor and didnt charge me any labor. And on my way home, it went off again.
Based on the earlier posts, I'm thinking that it's an electrical problem. I'm paranoid that if I take it back, they'll say it was another O2 sensor or maybe the speed sensor, or maybe the harness. Between the timing belt, spark plugs, and the O2 sensor, I've already spent $1200.
It has to be something that they did when they changed the belt and the plugs, because the lights have never come on before.
Does anybody else have any suggestions?