I am not aware of the ultimate limitations or capabilities of the piezo-electric effect, but if a high explosive were combined with a piezo-electric substance like barium titanite it might be able to generate sufficient voltage upon impact with a target to create a lethal current. This would render armor plate totally vulnerable.

Wouldn't the vehicle act like a faraday cage? No matter how much electricity you zapped it with the occupants would be safe and sound.

It i difficult to assay what effect a strong electrical charge would have on a vehicle with vulnerable occupants, electrical components in its control and drive systems, and explosive fuel tanks. Aircraft have been brought down by lightning strikes. It seems to me it would be worthy of a test since success would be a valuable adjunct to military weaponry.

Tanks, the occupants, and the electronics within are hardened against this very sort of attack.

Many modern armored vehicles have armor composed mostly of ceramics and synthetics which are non-conductive. Most armored vehicles are designed to be able to safely run into high-voltage cables and such. Most armored vehicles also use diesel fuel which is pretty much impossible to ignite in a fuel tank. To have much chance of doing anything, the weapon would probably need to be about as powerful as an actual lightning bolt, which is not feasible. Anyway, good old fashion high-explosives can still get the job done.