As it takes little provocation for me to fixate on Giorgione, I have devoted some time this evening to the question: did Velasquez see Sleeping Venus before he painted the Rokeby Venus?
I'm not aware of any firsthand account of Velasquez commenting on Giorgione's painting or of his viewing it. Girolamo Marcello, who had owned two other Giorgione canvases, commissioned Sleeping Venus presumably in order to obtain “a talisman to guarantee Morosina and Girolamo an heir.” Though the painting is no longer at the Casa Marcello, the reproductive initiative seems to have been a success as it was Giralamo's descendent, himself called Count Giralamo Marcello, that in recent years put up Joseph Brodsky in Venice for which he became the subject of one of the Nobel Laureate's poems. Titian's Venus with a Mirror (below) was in the possession of the Barbarigo family during the time of Velazquez' visits to Venice. As Velasquez was both a famous painter and wealthy enough to amass his own collection of Venetian masters, it's not inconceivable that he visited these houses although not as inevitable as if they had been in museums.
Among the revelers at the 1694 Venice Carnivale was Augustus, younger brother of Johann Georg IV, Elector of Saxony, on one of his extended visits to the island town. Shortly after the festivities Augustus received word that his brother had died of smallpox and that he would become Augustus II, Elector. Over the course of the next few years Augustus, who would later become King of Poland and Grand Duke of Lithuania, bought up Venetian paintings for the Dresden palace, where Sleeping Venus hangs now, having been stashed away during the World War II bombing raids. Julius Caesar Augustus was believed in his time to be a descendent of Venus and was a patron of the arts, which may have affected Augustus II's resolve to obtain the canvas.
Though there is no documentation of Velasquez visiting the Casa Marcello, recently notes have been discovered stating that during one of his trips to Italy, he chronicled the idea to paint Il Pordenone's Venus (right) in reverse - a close rip-off of Sleeping Venus with a red blanket underneath by a Giorgione disciple. The fact that he's so inspired by Pordenone's version indicates that Velasquez didn't, in fact, ever see Sleeping Venus, but was inspired by it indirectly.
Some suggest that the Rokeby Venus was an inspiration to Manet's Olympia, but there's no evidence that Manet ever saw the painting in person, and no reference of Manet's to a reproduction of it that I'm aware of. The Rokeby Venus was in a private collection in Yorkshire, which Manet never got to, from 1813 to 1905. This is a case of Velazquez and Manet both riffing off paintings that were inspired by Giorgione (by Titian and Pordenone) and coming up with a similar end result.
I was oblivious to Velazquez having seen a knockoff of Sleeping Venus when I shared my thoughts the other day about interiors and exteriors. Velazquez, first of all, was not a landscape painter as there was no admired tradition of landscapes in Spain at the time he worked save for El Greco who was not considered an influence, and since nudes in Spain were then painted for noblemen's bedrooms, perhaps he was disinclined to turn over a new leaf for such a client. The interior also makes the mirror more credible, enabling the portrait of the face which is essential to the effect of the work.