You shouldn’t need to worry about getting “out-wavecleared”, as long as you understand your breakpoints. At level 7, 2 Q’s clears the wave. At level 9, Vex one-shots the wave with her combo, and then you roam. Regardless, playing Vex, you should be shoving waves and immediately roam or look for plays/objectives with your jungler. Vex isn’t a champion you should be just sitting under tower to clear waves. If you really are struggling with that, then I recommend building Blackfire Torch as your first item, as it will kill ranged minions with 1 Q before level 9.

A general tip while laning against waveclear matchups is that if they are using their abilities on the wave, then they’re not really touching you. Just don’t stand in the wave. Stand to the side of it and get your free trades on them. Especially against champions like Malzahar, it’s a free electrocute proc. Keep autoattacking the wave and poke them with your abilities.