Tell me you mean that you have played 1 & 2, just not on the steam deck.

Anything otherwise would be a shame.

For those who haven't played the series, VATS is an alternate aiming mode where one can pause (or in later games in the 3d series, greatly slow) the game, select a certain number of targets depending upon available action points, and then have all those shots taken in rapid succession, with the game aiming.

I'd say that VATS is kind of a "path" than a purely alternate input method in those games; you need to make a VATS-oriented build, though it's true that it makes it possible to play the game with minimal FPS elements. Like, in Fallout: New Vegas, VATS provides major benefits close-up. While VATS is active, there's enormous damage reduction applied to your character, IIRC 90%, so for short periods of time, they have enormous damage output and little risk. They can also turn rapidly and target multiple enemies, probably better than a player manually-playing could. At close ranges, VATS is just superior.

But VATS suffers severe accuracy penalties at range. Whether-or-not a target is moving doesn't affect VATS accuracy, but range does a lot, whereas with manual aiming, whether-or-not a target is moving makes a big difference and range doesn't matter much. As a result, VATS isn't great for sniping, which is also an aspect of the game. You can do it (especially, oddly-enough, with pistols, in Fallout 4, where the Concentrated Fire perk lets later shots in a flurry of pistol shots at range be very accurate.

In Fallout 76, VATS provides such dramatic damage benefits that I'd say that it's impractical to play a non-VATS build -- VATS is required to get damage up to a reasonable level later in the game.