Engagement range is 1" horizontally, and 5" vertically (a cylinder, basically) . Since most flying vehicles are less than 5" above ground level, you will always get a successfull charge against them if you move within 1" from their hull's "projection". If the vault is higher than 5" above ground, you will have to get within 1" of its base.
Edit. Appartently the vault's stand is exactly 5" tall, so getting within 1" of it's projection will suffice.