With 8 dice hitting on a 4+ you should be destroying the vehicle.
Assuming the vehicle was undamaged to start with, you've "only" got a 64% chance of destroying it. But, as you say, you're virtually guaranteed to force it to retreat, and you've got an 86% chance of at least heavily damaging it. I assume that a Heavily Damaged vehicle cannot retreat and would therefore be destroyed, the same as a Disrupted squad... but I actually can't find any verification of that fact, in the rules, FAQ, or RCD! Interesting...