Taking down a VTOL can take a lot of ammo but a player on Reddit discovered a trick that can help you take down the airship with just one bullet.
You can throw two Thermites directly into the VTOL and they will stick to the airship and deal ridiculous amounts of damage. Follow up with a single shot of any weapon and the VTOL will explode.
You need to have a Perk Package with the Resupply Perk to pull off the trick because the Bonus Perk allows you to gain access to an extra Lethal.
Switching to an anti-aircraft loadout can be too much of an ask for some players and the Thermite method is much more efficient.
If you do not want to use the Thermite trick, th SO-14 can be one of the best alternative ways to take down a VTOL.