There is no problem using IdleMaster with VAC games, it can't trigger a VAC ban as it doesn't modify game files nor it gives any advantage in the game.
Also, I tested what I said in the previous post, I opened multiple ready to drop games and it dropped cards only for the first one open, at least now we can play while dropping cards ;)

A friend of mine told me about this, and I tested myself and it worked, I opened idle master and left it there idling a game to drop cards and went to play CS:GO, I played a match there and when I closed the game a card had dropped, maybe steam changed something on how multiple games running are handled? Maybe now we can drop cards on multiple games at the same time? This needs further investigation and testing.