A little confusing as reflective suggests the obvious but that vest offers so little area to add graphics the value of reflective is questionable. Possibly more for identification / ownership but I don't see the territory for effective , reflective placement.
Stahls makes vinyl specifically for nylon, I would think putting this down on the nylon is your best opportunity for success. Not sure you can get it in reflective but if not possibly then add the reflective over the first down nylon film.
Second thought, add the reflective to a piece of canvas or similar fabric then applique' sew the canvas patch to the vest the back panel area looks like your best opportunity for the applique' method.
mooseman