The US carriers all provide some degree of automatic upgrades to frequent fliers on domestic flights, but on international, they work like VS - either you pay (in cash, miles, or some other instrument) or (rarely these days) you get an op-up that is solely at the airline's discretion. The major US carriers have fine-tuned their revenue management software so that op-ups are less and less common.

The route you fly, the day/time of flight, and the aircraft are all factors in the op-up roulette. A flight on a VS A350 that has 44J 56W 235Y has statistically better op-up odds than a VS A350 with 16J 56W 325Y. Similarly, some flights have higher odds because of the pax mix on the route. I had a fantastic run with op-ups on DL 15ish years ago flying between ATL and LHR because I studied the pax mix. ATL-LHR was (and still is) a business-heavy route and DL can easily sell every seat in the front. But MSP-LHR was a leisure-heavy route and often oversold in Y. So by booking ATL-MSP-LHR I greatly improved my op-up chances. I had a 50% op-up rate for about three years, which is pretty incredible. It's much more difficult these days because of software advancements. It's also going to be inherently more difficult on VS because there is only one hub.

As others have said, op-ups are great but you can never count on them. You're usually better off just buying a ticket in the cabin that you want to sit in.