Is the range lower than without the ability? It just says that it cant be used on that target. It was visible. Never worked for me though.
The ability doesn't actually target a vehicle, but a location within range. Said range is actually extremely short but marked out by the dotted circle perimeter on the other side of the fence, as you can see in this picture.
Attempting to target anything outside this circle will just give you an error message, as they are outside the range.
All in all the direct fire's basically a flavour/gimmick ability at this point, with a short stun and it's barely even 1/3 of a med tank's hp. If they're that close your LEFH is dead anyway. Might as well try to blow them up on approach as you can target anything outside the direct fire range (but inside normal barrage range)
EDIT: And yes all the trucks died in this testing.