You forgot that maul slipped on off-screen banana's pelt there
As for Maul vs Vader fight, I'm pretty sure that it will happen, as it is huge fan service - thankfully, it wasn't spoiling that last episode, will have to be silly somewhere else. Albeit, Vader shout actually slice him to small morsels, if only to ensure that maul "won't be back for the sequel". At least, that is what is expected when ensuring death of someone who was cut in half, thrown into pit of planetary generator (two pieces separately), sidious'ed, and left to rot for years on planet without any food/water sources.
...actually, disintegration on atomic level might be better idea.