I don't expect it to be that concerning. DMs already roll saving throws all the time when casters use cantrips, so on the off-chance there is a player at the table with a weapon that has Topple, that's just one more saving throw. And it's not even every attack in the round, because once you use Topple you won't be asking the DM to roll any more saving throws again since the target is already prone.They are going to bog down combat especially the one where the DM has to roll to resist knockdown every hit and giving yourself nearly permanent advantage is a must have so there goes your level 4 pick.
And as far as Vex for the "permanent advantage"? People already say that the lightly-armored, ranged DEX characters are way too powerful and that everyone always uses them because it's just "the best"... so who cares if they shoot their bows with Advantage now too? Making 'The Best' even 'Bester' doesn't really gain you a whole lot.