REALLY depends on what the VaM scene is like and if the creator of that scene bothered to add ways to interact with it on desktop.
Lots work fine, but there's situations like one scene I tried where there was this lever you had to grab and pull to switch the device on.
In VR, the lever swings like a real lever, activating the machine.
On desktop, ALL the grab mode options, every possible trick you could to to try to flip that lever on, would either do nothing, or the entire lever assembly would move.
That's a bit of an extreme example though.
More often, there's no huge glitch like that, what I encounter more are things like trying to get a toy to the right angle to work, having a hard time getting a good camera angle to see things properly, that kind of thing.
Actually, specifically the HuCow game as a pretty good example: milking and the bucket is a little clunky since in VR you hold the bucket in one hand and her tit in the other, but in desktop mode you only have one "hand" and so you just kinda levitate the bucket NEAR her boob and then you milk her and try to get as much of it into the bucket as you can. Also, dumping the bucket out into the collector thing is pretty clunky too since you can't just grab your mouse and rotate it, I think VaM's got a "rotate your 'hand'" button somewhere, heh.