Snoke and Luke are top tier as far as raw Force power is concerned, but Luke is extremely rusty and Snoke is waaaay out of his prime. Valkorion is at least nearly as powerful as Luke and Snoke, if not far beyond the both of them in terms of power in the Force.
Once you factor in physical ability, Snoke becomes more of a handicap to Luke. Valkorion's ability to casually block lightsaber strikes with his bare hands gives him a huge advantage, although Luke is almost certainly the more skilled lightsaber combatant here.
All in all, Valkorion almost casually takes this. Snoke is too frail to survive an engagement against such a ridiculously powerful adversary, so I'd say he'd get taken out almost immediately. Leaving Luke vs. Valkorion. A hermit who had cut himself off from the Force for years, and as far as we know, hasn't had a serious lightsaber duel in decades, against a Sith Emperor who has been active for centuries. It's just not even a contest.
Valkorion atomizes both of them without a gesture.
__________________
"You were weak when I found you. I did not expect you to survive your training. But now, your hatred has become your strength. At last, the dark side is your ally. "