Try to avoid mixing the vet. Due to the way it stacks, you get better bonuses for sticking to the same vet type.
If you are going Tankdestroyer putting triple offensive vet on your ATHT can be awesome together with the APCR Rounds passive upgrade, it will penetrate the shit out of allied tanks.
No on ever said APCR applied to shrecks . IIRC they only apply to Marder, ATHT, Panther, Hetzer, Jagdpanther