Vinyl needs the J-channel for a couple of reasons...primarily to hide the un-eveness of the ends of the vinyl claps as they move with temperature swings and give a somewhat clean line...and...well, I really can't endose it as a flashing, because it isn't. So I guess there's only one reason.
Vinyl is not tight to the weather. In rain with winds, water will be driven behind your vinyl. It happens, and everyone knows it, from the manufacturers t the installers to the homeowners who had vinyl installed directly over the sheathing and had rotted sheathing after 3-5 years. Vinyl is not water tight.
Thus, you need a drainage plane behind the vinyl that will shed water that gets between the vinyl and the house's sheathing, because get in there it will.
Your drainage plane can be a spun-poly housewrap or tar paper, PROPERLY tied into and overlapped with the fenestration flashing. Some use foil-faced polyiso insulaton with the seams taped. In renovations, some leave the old siding and depend on that to hold out the water, just using the vinyl to dress things up a bit.
Just remember, the J-channel is NOT flashing...it's a decorative trim piece.