Fall or spring should be fine. Don't add miracle grow.

You could plant sweet woodruff among the vinca. Its fluffy white flowers look amazing against the purple flowers of the vinca. Once established, sweet woodruff spreads like crazy.

Vinca looks good all the time, Sweet Woodruff doesn't, and is too aggressive for me. I can handle Vinca minor by planting it in appropriate places. Sweet Woodruff spreads by seeds as well as runners and pops up all over. Mine came in with some Ajuga. They bloom the same time.

To move Vinca I cut out one foot circles from an established patch and then refill the holes. In wet, cool weather, of course.

Mike