If attacking the vehicle from the flank, an infantry unit would need a 12 on the dice. Improbable, not impossible.
Moving to Short Range (still flanking), they need a 9. More probable.
Moving to Close Combat, they need an 8. More probable.
With a well-placed Action Card it's even more probable to hit and subsequently destroy the vehicle.
So don't give up just because you have infantry
You can wait for the vehicle to become Spent, and then move into close combat (if you're willing to risk a potential command card coming up).