According to NGA and the Washington Post, all advance tickets for the Vermeer
show have been given out.  Your only bet is to show up at the museum at least
an hour or so before opening and stand in line for same-day passes for use
later in the morning or afternoon.  You probably won't have much control over
the time/hour of your ticket.  The good news, however, is that the show's
hours have been extended into the evening, until 9:00 p.m., to accomodate the
crowds.  Good luck!  It's well worth the struggle to get in.