A normal torpedo hit must first roll to get past Armor. If successful, it inflicts a point of damage on the target. Once the target has suffered a point of damage, another roll is made to see if that damage causes any critical damage - doing so on a score of 6.

So, an example:

I fire a Str6 salvo at your cruiser with 2 Turrets and 5+ Armour. Let's say one turret hits, meaning that 5 torpedoes now attack. I roll 5 dice against your armour value. Assuming I roll well, let's say that 2 dice score 5+ and the rest miss. Your cruiser would then take 2 points of damage. I then roll two more dice (one for each hit) to see if critical damage occurs. Each '6' I roll would then result in one roll on the Critical Damage table (rolling 2D6 and adding it together).

Vortex torpedoes would follow the same process, except that at the end, instead of requiring a roll of 6 to generate critical hits, you would just proceed to make two rolls on the Critical Damage table.