Halak allowed four goals on 16 shots before exiting early. He started for the second consecutive night after the Bruins dropped Game 2 in overtime on Tuesday.
Tuukka Rask, Boston's longtime starter, left the bubble earlier in the playoffs to deal with a family matter.
Vladar, 23, was drafted by the Bruins 75th overall in 2015. He had a .936 save percentage along with a 1.79 goals against average across 25 games with Providence's AHL club this past season, while also making one start in the ECHL.