I couldn't get it to work on my 09 WRX. When I hold down the VSA button when I turn the car back on, it just turns on a the yellow light like it would when it is off, and then a few seconds later the light turns off. When I press it again after that nothing happens. I actually can't even turn the VSA back off and on after that unless I restart the car. Maybe it's a different procedure for the WRX trims.
I've gotten used to the hill assist honestly. It holds a little bit longer than I would like, but it's a pretty minor annoyance that I have to deal with like two or three times a week at most. Still, I'd like to turn it off if I can.