All the references seem to spell it as you do. My guess is that it refers to any medical procedure which is performed by accessing the retina by going through the "vitreous humor" which fills the cavity behind the lens (of the eyeball.) Many references mentioned "fine needle biopsies," which would extract material through a hollow needle, to be examined in the lab, presumably material from the retina. There are references to the "scleral entrance site." The "sclera" is the outer covering of the eyeball.
"Trans" usually means "through" or "across." The "transvitreal route" is often mentioned, so that might mean through the lens, and through the vitreous humor to the retina.
You probably should check a medical dictionary.