Remember that under my suggestion, we're talking a total of 6 months that a sleeper vandal would have to "sleep" before they entered the ranks of "normal" admins (3 months of keeping your nose clean, followed by three months of continued good behavior as an admin). And remember, someone setting out as a sleeper vandal would not be guaranteed of becoming an admin after three months, it's still up to the random selection. Is it impossible for someone to game this process? Sure. Is it likely? Hardly.
The flaw in that logic is in thinking that there is anything different between a "normal admin" and a "provisional admin" besides the name. There isn't. Adminship is a bit in a database, either you have that bit and are able to do enormous damage to Wikipedia, or you don't. There is no need to sleep for 6 months when you can do the damage immediately. It would take a few minutes at the very best to desysop a provisional admin that started vandalising - how much damage do you think an AdminVandalBot could do in that time? I imagine it would be hours normal-admin time to cleanup.