Is taking an old prescription (vyvanse) legal? could i be fired for a positive drug test, even if i have the prescription for it, though it is old?

Complicated: Yes, depending on the company's policies, you could be fired - even if the Rx was current. You'd have a good fight if it was, but you could still lose. Being out of date, your case would be very weak if they wanted to be tough. If you took it for the medical purpose, I feel bad for you but it was still a risky idea. If you took it for fun - it was more foolish. You can learn and hope for the best.