Vermeer made only low dozens of paintings as opposed to, say, Monet, who made thousands, so likely they just mean something like “including one of Vermeer’s paintings, which are rare”
Many of the “majors” had a factory in which the “master” or headline figure might sketch out a painting, have it filled out by someone else, and then perhaps be finished off by the master. Sometimes multiple versions of the same painting were produced.
The Vermeers and Van Goughs didn’t have all that infrastructure (and Vermeer painted little, or at least little survives)
Vermeer is the "shiny Pokemon card" of painters. Some museum recently announced they'd gathered a temporary collection of 20-odd Vermeers. Tickets for the show were sold out within minutes - a year ahead of time. Normally if you wanted to assemble a "complete collection" of Vermeers you've seen, you'd have to travel to like 10 different countries. Guys online talking about having no hesitation paying for a ten hour flight and two-day vacation for the sole reason of seeing this exhibit.
I dont know about the details of this particular painting, but Rarity here could refer to something like the type of paint used, the size, or even the subject matter or time period.
What does this mean? Wasn't Vermeer a painter, and therefore all of his works were one of a kind (unless he painted copies)?