A tree peony for sure......a herbaceous peony, I wouldn't advise. Both are very vigorous evergreen groundcovers (although the vinca much less so in enough sun to keep peonies happy and blooming....it prefers shade) and can grow rampantly enough to overwhelm or smother herbaceous perennials. And you will have enough bother keeping them from growing up and into the tree peony anyway :-)

I'd try something like Veronica peduncularis/umbrosa 'Georgia Blue'. Can take as much sun as you can throw at it but will also take some shade. Very low growing, evergreen (with bronze-burgundy winter coloring) and with bright blue flowers that will compliment any peony. Mine blooms heavily in spring but then sporadically through summer and into fall.