Welcome to the site!
Daughter had rock mulch in her front garden of the home she purchased. She offered up the rock mulch for free on a site, it was gone within a few days.
Daffs like good drainage, if you are unable to give the gravel away, I would try to take out half of the amount, mix the remaining amount with the soil and plant away.
Like Stone said, Vinca can be invasive depending on its site, I have vinca in Zone4B and it stays very contained.
