My suggestion:
Unless armed with "anti-vehicle weapons" like hand grenades (and those would in all probability rather be thrown), I´d think regular infantry can´t really damage a vehicle.
If the vehicle is open-topped, though, I´d allow a unit to charge a vehicle moving up to speed 1/6" tops, and then fight a single round of melee against the passengers, the latter counting as having cover. You only conduct a single round since the vehicle will then move on. If the vehicle is stationary, you may continue the melee until it is resolved.
On flight moves for a vehicle unit losing a melee, as noted above, if moving, it will automatically move out of the melee after the combat was fought, with neither side taking a morale check. If the vehicle is stationary or immobilised, take normal checks as appropriate.
As said, my personal opinion on how you could tweak the rules to account for anti-vehicle melee. Björn may have a different opinion, and mine is in no way official.