First, I doubt very seriously that a single torpedo could break the back of a VLCC. It's just too big and built too stoutly.
Most newer tankers are double hulled, for instance. They are built tough to avoid oil spills.
As I noted, I wasn't talking about just strapping missile systems to the main deck: the ships (including propulsion) would have to be thoroughly overhauled, redundant power and command centers added, comms, and, of course, CWS. Outer bulkheads would have to be reinforced, perhaps some armor added.
Such ships would have one overwhelming advantage compared to regular Navy ships when it comes to weapons and armor: size. Heck, you could put in more bulkheads and just fill the outer compartments with water and you'd still have an enormous interior space.
Stuffed with long range land attack and ship-to-ship missiles, as long as it was within range it would retain major offensive capability even if seriously damaged.
As for torpedoes, consider what two torpedoes did in 1982 to the American-made WW-II era Argentine light cruiser General Belgrano during the Falklands war. The first torpedo blew off its bow, while the second struck about three-quarters of the way along the ship, just beyond the side armour plating.
That torpedo -- a pre-WW II model with 805 pounds of torpex explosive -- punched through the side of the ship before exploding in the aft machine room. The explosion tore upward and ripped a 20-metre-long hole in the deck, also taking out the ship's electrical power.
The ship rapidly filled with smoke and water flowed in through the hole in the hull. Due to the electric power failure, water could not be pumped out and the General Belgrano sank within hours.
Exploding a torpedo under a ship is said to do about ten times as much damage as a simple hull hit. The bubble of explosive gasses lifts the vessel, then drops it, making the vessel collide with the water rushing back in. This usually breaks the ship's keel, opens seams along the hull, and does major internal damage. Under such a pounding, ships often sink, frequently breaking apart as they do.
Might a supertanker with its bulk, multiple tanks, and even a double hull survive such an attack? Perhaps, but there has never been a direct test of the proposition, and I am hard put to think that a supertanker would do better than a substantial naval warship. Even if a supertanker survived an initial salvo, it will be damaged and can be hit again and again until it sinks.