What is this?
It's a stained glass window dating to circa 1869, designed by Henry Sharp. Sharp was based in Manhattan and designed windows for churches up and down the East Coast.
Why are there stars of David as well as a Maltese cross on this piece?
Because the Christian church accepts the Old Testament as a pre-cursor to the New Testament, the Star of David you see was an acceptable symbol to have in a stained glass windows. These motifs were used often in stained glass windows in churches and other public spaces in the mid 1850s when stained glass was beginning to see a revival in the US. Sharp designed windows for churches all over the east coast. The Met Museum actually has another one by Sharp that has similar motifs to the ones you see there.
What is vitreous?
Vitreous basically means glass. Vitreous paint has a chemical make-up similar to glass and can therefore bond to the glass of the window itself. It makes the pigment more durable.