Currently not all vehicles that could reasonably support FFV for crew positions have that ability added. The following list shows some vehicles that could use the FFV feature.

Wheeled
AFV-4 Gorgon -
rear passengers?
AMV-7 Marshall -
gunner, although it might be tight with the commander camera in place.
rear passengers, though it seems the hatch on the interior doesn't match the exterior.
MSE-3 Marid -
commander, would require the hatch to be opened more.
rear passengers.
Rhino MGS -
gunner, apparently this is already planned
commander, apparently this is already planned
Strider -
Commander, has an unused hatch above the seat
Passenger, has an unused hatch above the seat

Tracked
2S9 Sochor -
Gunner.
AWC 301 Nyx (AT), (AA), (Recon), (Autocannon) -
Commander, apparently this is already planned.
BTR-K Kamysh -
Driver cannot turn out at all.
Gunner cannot turn out either.
Commander cannot turn out either.
Passengers.
CRV-6e Bobcat -
Commander, would require the hatch to be open more
FV-720 Mora -
Gunner, commander camera may block
passenger
IFV-6a Cheetah -
Gunner
Commander
IFV-6c Panther -
Commander
M4 Scorcher -
Gunner
MBT-52 Kuma -
Commander.
T100 Varsuk -
Gunner
Commander
ZSU-39 Tigris -
Gunner,
Commander