One way to look at it is to look at how much damage Ifan does when he doesn't back stab thanks to Vault sometime putting him in front or to the side of the enemy and you forget to reposition him.
Can you still clear the early access without it? Likely because in the current early access, weapon damage are ridiculous so using Chas and Abs unique knives for the early game to whatever Gareth have for sale, you still deal great damage even without backstab.
But if you can't backstab with your knife (which is required to use vault), you might as well just pick up a bow and shoot with it because that's pretty much the damage non-backstab knife will do, but with range and special arrow. The backstab is the only reason I don't just change Ifan into a bow user.
