Personal scale weapons can't do any damage to vehicle scale targets.
Even the Missile Tube (the most "powerful" weapon) can only harm vehicles with Armor 2 or less, but can't do nothing to vehicles with Armor 3 or more (like an AT-PT).
I was thinking about a Quality (for personal scale weapons only) like this:
Anti-Vehicle: Every success gives +5 damage against Vehicle scale targets (instead of +1 damage).
So, every 2 successes, it would increase the damage done against vehicles by 1 (since 10 personal scale damage = 1 vehicle scale damage).
This Quality could be added to all personal weapons that have the Breach Quality.
What do you think?