As the rules state a non-walker vehicle that has not moved in it's previous movement phase (or is immobilized) is hit automatically by melee attacks.
Skitarii "Taser" weapons and Gabriel Seth cause additional hits on a to hit roll of a "6" with their melee attacks.
Would you still roll "to hit" and get additional hits from the 6's or just bypass that rolling and miss out on those extra potential hits?