My understanding is that the vesting started when he left Google/Waymo. Uber made vesting retroactive back to Otto's nominal start date. This has been called out by Waymo's lawyers.
Of course, if it is the case that Otto was built on misappropriated trade secrets, and Levandowski lied to Uber about that during the acquisition, then we might well see some of that clawed back in a separate lawsuit.
From what I recall, that purchase was made in Uber stock, and I'd expect it to have a vesting schedule. Depending on the length of the initial cliff, it seems possible that he hasn't vested any of that stock. I am speculating though and don't know for sure.