I know the Sharks aren't going to sit Vlasic and aren't going to play him in a heavily sheltered role. Sports teams don't do that with expensive, veteran players unless its become beyond obvious that the player shouldn't be playing.
But that's a problem. Vlasic's contract is a sunk cost that, odds are, the Sharks are not going to be able to get out of - they can buy him out, they can hope that another team will want him without demanding a ruinous price and that Vlasic will be interested in moving, they can hope that the league will offer a cap-free buyout at some point, but odds are only the first of those is plausible, and it will hurt.
Because Vlasic's contract is a sunk cost, the Sharks should not be worrying about how much it'll suck to have a $7 million backup defenseman for the next six years. If the Sharks have six defensemen who are better than Vlasic, they're the ones who should be playing, unless by playing Vlasic the Sharks think they can convince someone to take him off their hands, but that may be unlikely.
Now, do the Sharks have six defensemen better than Vlasic? Probably not right now - neither Pasichnuk nor Merkley are proven - but by mid-season it may become clear that the Sharks do have six better defensemen than Vlasic. They won't bench him, but if he continues to play like he has, they should at least give it serious consideration.