VSC light flashing at higher speeds

08 Prius - 143,000 miles. I just picked this used '08 up yesterday. During the test drive the check engine light came on. Service traced it to water fouling a wire from the detailers cleaning the car/engine. They replaced the wires and the engine light has not come back on. On the way home when I hit 65 mph the VSC light started flashing and the car would not go any faster even if you mash the pedal. It would shut off when I let off the gas but come back on when I gave it gas. Once I got onto another road doing about 45 mph it did not come on. When I hit 55 mph a little later, flashing again. Today, a day later, the same thing happened when my daughter drove it. Called the dealer (Toyota dealer/service). The service manager says it could still be related to the water that caused the check engine light. He said give it two more days (to dry out) and if it's still doing it to bring it in. I also noticed the manual says to keep tires matched for proper ABS and VSC function. I know the dealership replaced 2 tires. Could the problem? If so, why wouldn't it be a problem at 40-50 mph too? Other thoughts on what it could be? Googling it makes it sounds like it could be related to just about anyting in the car!!
Thanks.