There seems to be an awful lot of fluffing around the issue here. The airside driver must have an airside driving permit and vehicle RT certificate.
If he has crossed a holding point (his training would have taught him what this is) then he MUST NOT re-enter the runway without clearance.
It is of no relevance that the tower controller should have foreseen a potential problem (there is no requirement to) and there is also no requirement to report vacated. Having said that if a vehicle is cleared to operate on the runway it is good practice to report vacated but the fact that the tower controller saw him cross the holding point makes it perfectly acceptable to clear an aircraft to takeoff/land.
Grass roots fact is that the driver is at fault (it's a runway incursion). MOR it, accept it and learn from it.