Vasilevsky isn't playing behind them and has been below average this year. he's got a .899 sv%.
I think the idea that the team in front of him is the reason Korpisalo has been inconsistent just doesn't line up with reality. I mean, Forsberg as you put it, had vezinal level numbers one season, that year we had an xGA/60 that was 6th worst in the league (all strengths), this year our all strength xGA is 13th best in the league, and it's been the best in the league since 12 Jan, unfortunately that's been paired with league worst sv% all year (and since 12 Jan)
We've been getting team defense good enough for a goalie to look acceptable, our issue is no longer the play in front of the goalie, it's been on the goalie for a while now.