You don't polymorph into anything with a template, just a Vanilla kobold vampire. The Spellstitched Template (MM2) says that if a caster is undead, he can apply the spellstitched template to himself, for 1000gp, plus 1000xp, plus 500xp for every point of wisdom the creature to be spellstitched has. With leadership, your cohort can apply the spellstitched template to you (thus taking the XP hit), afterwards you can dismiss him and gain another cohort with full-xp. Making him a Vampire afterwards.