There is no way to spawn The Dawn Maiden without taking some hits to your faction with The Guardians of the Vale. If she doesn't spawn, you can grind faction back up while waiting for the placeholders to repop.
Kill the groups of vale maidens that are static spawns under the trees. The wandering ones don't seem to be part of the spawning process.
If you're lucky, The Dawn Maiden will spawn with one of the static groups.