i'm very sure it's not possible to change the vet bonuses on the halftracks. i think a better way to deal with the problem is to make the vet combat oriented and leave it at that. as a reinforce point the halftracks don't benefit significantly from vet.
The reinforcement radius vet was pretty damn good when it was actually obtainable and usable. It helps to keep it alive and easier to micro when battlefields get crowded with tanks - therefore, I think the concept of reinforcing halftracks earning vet from nearby units fighting is one worth trying.