Vlasic left for the Sharks dressing room five minutes into the second period.
It was unclear if his absence was due to an injury or equipment-related but it strikes a blow to the San Jose defence no less.
Vlasic makes up one half of the Sharks top defensive pair, alongside Justin Braun. The pair are tasked with defending Sidney Crosby and the Pittsburgh Penguins top line in the Stanley Cup final.
Crosby set up Pittsburgh's second goal of Game 6 shortly after Vlasic left the game.
The 29-year-old Vlasic played almost 28 minutes in a Game 5 win and has been fairly effective in slowing Crosby.
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