I see their relationship quite the opposite. Vinny is a condescending asshole who constantly puts her down. She definitely has her insecurities and weaknesses, but he preys on those and tears her apart, then offers some half-assed encouragement like "you can do it babe" when it's already too late.
She can't drive a stick, which I think is a fair criticism, but half the people in the history of TAR have lacked this skill. Her navigation skills seem adequate. There are definitely teams that are worse, and it doesn't seem like poor navigation has hurt Amber and Vinny at any point so far. Even other "good" teams have struggled with navigation, but they don't destroy each other over it.
As far as completing challenges, Amber has mostly been fine and it's very difficult to do anything well when your partner is constantly telling you how stupid and shitty you are. Imagine spending a day with your spouse, having to assemble things, navigate a foreign city, perform music routines, and your wife spends the entire day calling you pathetic, slow, and stupid, all on television for the world to see. How can anyone be expected to do well in those circumstances? She should leave his ass.
I don't even particularly like Amber, but I think Vinny is a narcissistic piece of shit. If they split up, I would fully expect him to find another insecure and emotionally vulnerable partner and treat her like shit.