This year, Vokoun is 3-0 with a .970 save percentage and 0.90 GAA against the Islanders. That's quite good.
Last year as a member of the Capitals, Vokoun was 0-2 with marks of .887 and 3.52 against the Islanders. That is quite bad. In 2010-11 with the Panthers, a far worse team than either the Capitals or Penguins, he was 2-1 with .927 and 2.68.
Small sample sizes are deceptive. Over the last three seasons, Vokoun has been really good, really bad and mediocre against the Islanders with three different teams (with one good, one bad and one mediocre team, no less).
When the Islanders take the ice in Pittsburgh tonight they're going to be up against a netminder who hasn't played in two and a half weeks, since April 22nd to be exact. It'll also be against a goaltender who was brought in as a high priced backup for this exact type of situation. In June 2012, the Pens traded for the rights to Vokoun and promptly paid him $4 million over two years to have him loom over Fleury's shoulder. Now they'll have a chance to make the investment worth it.
No one is really going to know how Vokoun will play and react until we see him on the ice, but chances are he'll be better than Marc-Andre Fleury. OK, to be fair, that isn't very hard to do at this point whether Vokoun continues to have a .970 save percentage against the Isles or not.