This is actually possible on both accounts! It takes some slick programming but its possible. It also isn't likely to adapt any 'normal' T-bar to vMix, because vMix's T-bar doesn't mimic physical T-bars, it's more like a 'one way only' slider: once you reach the right end, inputs will be switched and the T-bar is reset to left side - no physical device can do that.
To the best of my knowledge, there is no hardware T-bar available, that would work with vMix.
