help- finishing w/ single crochet before or after

[loveswildflowers]

Hi! I am have knitted a vest. It is still in 2 pieces (front and back). I want to single crochet around the armholes and neck, but I am wondering if I should stitch top and sides of the vest together first and then go over it with the crochet - or if I should do the crochet first and then sew? I am such a newbie!!