In vanilla, villagers in minecarts/boats that get killed by zombies are converted to zombie villagers that are also in the minecart/boat.

That behavior is not mirrored by this plugin.

Villager in minecart getting attacked:

Free zombie villager after getting converted:

Fixed in #11

a possible solution would be to check if tVillager.isInsideVehicle and if so, Entity vehicle = tVillager.getVehicle and vehicle.addPassenger(zVillager) after removing the old villager from the vehicle.

I don't have the means to implement and test it right now, but by looking at the decompiled source for minecraft, it does something similar.