Considering that a Warbike's armour is 18, 18, 14, I think 12 on all sides is WAY too low. 20 on all sides is perfectly reasonable. You really shouldn't be able to damage the thing with a lasgun and indeed, on the table top something like a Warbuggy or Sentinal is just out of reach of the maximum damage potential of a lasgun (Strength 3 + 6 on the d6 vs their armour on all sides of 10), just as an armour 20 vehicle is just outside of the maximum damage potential of an overcharged lasgun (max 15 damage and 2 Pen vs armour 20, so 3 points short).