Is it as simple as just ISA conversion for TAS/IAS ? I don't pretend to know the answer but a bit of quick research suggests its more complicated. A quick look at a 172 poh gave sea level and 10000 ft figures for Vx and Vy and those would be already in KIAS but I'm guessing that its a curve not a straight line between those two points and this is what the OP is looking to figure out.
Bit more here that helped me understand the question, if not the answer.
Vx, Vy and Absolute Ceiling | Jetcareers