It depends on the width of the new door jambs. You might need extensions or if you are decent at carpentry/woodworking, you can diy. If your jambs aren't wide enough to span the inside wallboard and outside sheathing , you could flush it up to the outside and fur out on the inside by extending the jambs. It also depends on the thickness of your trim. If its 3/4", the you should probably tuck the 3/8" insulation behind it. Thicker material such as 1-1/2", you could butt the 3/8" insulation up to it. The vinyl should have a "j" channel around the perimeter of the door trim that the siding slips into. That "j" channel should butt up to the trim but not protrude out past the thickness of the trim.