You need a creature for Squire's Devotion to target as you cast it. There's no way to have it enter the battlefield attached to the Vampire token it'll create.
2018-01-19
If the creature this Aura would enchant is an illegal target by the time Squire's Devotion tries to resolve, the Aura spell doesn't resolve. It won't enter the battlefield, so its ability won't trigger.
Enchant creature
Enchanted creature gets +1/+1 and has lifelink.
When this Aura enters, create a 1/1 white Vampire creature token with lifelink.
