it's not Vinyl J channel. it's Vinyl L bead with a rip away strip, it staples in tight to window, after finishing out to corner, you pull the rip bead out for nice smooth line against window frame. a tiny bead of caulk cleans it up nice against window or could use a paper tape on L bead
Exactly what I was going to recommend tear away bead. The tear away part keeps the mud from where you don't want it. Then when mud is dry tear the strip off and you have a nice straight line. Also a great product for anytime drywall meets another surface such as brick or stone. Lets you mud the edge of the drywall without getting mud on the brick, whice is a beatch to remove.