Hello Pi-C, I've ran some more test and actually the grid contents aren't lost when the vehicule is mined, the grid is preserved and its content aswell, when you place the vehicle again in the world the grid is still there and accessible!
The bonus is also properly reapply on the vehicule placing as its grid is checked again for its content. It's actually the same working as the modular armors, they keep their grid and content even if you unequip them.
Also, like the armors, you can actually open the vehicule grid from your inventory if it has content :
https://imgur.com/PSPuTI3
Let me know if you can confirm this on your side !