Penetration is worked out as normal, (D6 + the Strength of the attacker). Note that this means few troops can actually harm vehicles as even the lightest vehicle has an Armour Value of 10.
Monstrous Creatures. Monstrous creatures are so big and powerful that they roll an extra D6 to penetrate vehicle armour, so their Armour Penetration value is equal to 2D6 + the Strength of the creature.
Melta Bombs. Specialist tank hunting troops are often equipped with melta bombs – fusion based thermic charges which can reduce a battletank to a burnt-out wreck in milliseconds. Troops armed with melta bombs can treat any of the hits they score in an assault on a vehicle as close range melta gun hits (Penetration S8 + 2D6).
However, melta bombs need to be clamped in place before activating so each model using them makes only one attack and doesn't count bonus attacks for charging, extra hand weapons, etc. Melta bombs can be used in a Death or Glory attack against a tank attempting to overrun.
Krak grenades. Some troops are equipped with krak grenades, a special type of grenade designed to implode, shattering armour and (hopefully) stopping vehicles in their tracks.
Troops equipped with krak grenades can use them to assault vehicles. Krak grenades give troops S6 for close combat hits scored against vehicles, making their Armour Penetration scores 6+D6. Krak grenades need both hands to trigger so models using them can only make a single attack and don't get bonus attacks for charging, additional hand weapons or anything else. Krak grenades can also be used to make a Death or Glory attack on an overrunning tank.