I do not have anything against having to get out of vehicle and rearm it "by hand", but it gets very tedious as you can't pick up most of the vehicle ammo unless you have a backpack (which you don't most of the time since those are only found in HQ and harbors at the moment). I am not so sure how correct it is to require vehicle crews to have huge backpacks, realistically speaking.

This currently applies to resupplying a BTR with the 14.5x114mm ammo, I did not have the opportunity to rearm a Humvee, I imagine it has a similar issue, but it depends on it's ammo weight.

Example:
Rearming a BTR in Conflict game mode:

1. Open ammo crate at any Armory
2. Try to take the ammo
3. It's too heavy, even if you put away all your weapons

Workaround: Have a completely empty backpack, put one crate of ammo in backpack, then board the vehicle as gunner, press tab, put ammo from backpack to turret. You have to do this several times repeatedly because the turren storage is not accessible from outside of the vehicle.

Without backpack you are only able to put ammo into the trunk of the BTR by first putting it on the ground by dragging into "Vicinity" storage, and then into trunk, but there is no easy way to transfer ammo from trunk of a vehicle into the turret.

Possible Solution #1: In Conflict gamemode: make backpacks more available (at normal bases). Other game modes: crew needs backpacks. However I think this might only be a temporary solution depending on how heavy ammo for Tanks and Helicopters will be in the future - those may not even fit the backpack weight. Vehicle crew with big backpacks look weird though.

Possible Solution #2: Have a way to move ammo directly from ammo crate into the vehicle. Using the existing inventory system - for example by allowing to open vehicle inventory instead of personal inventory on the right side. Or by allowing to carry individual vehicle ammo boxes in hands (unable to use weapons unless dropped) physically - most immersive but probably requires more work.