they use up several vans in the productions... they have a show van, a speed van, and a crash van... the crash van is sometimes a van they used in another epp that was not totally crushed up but was damaged, so they keep those around for other epps... just look at sometimes when the van is jumping off ramps (a la general lee in the dukes of hazzard) and you will sometimes see a different model than what was in the scene prior to that...
also as for the vette, they rented a vette only for those scene and the stripe had to be applied to the vette, so they didnt always have the same vette...