Your first step would be to get rid of all the plants you do not want to remain on the slope. This can be done by spraying with a general herbicide such as Roundup. Roundup only works on plants that are actively growing, so you must do this step before frost kills tender species. Then you can plant wildflower seeds on the slope and then cover the slope with a biodegradable erosion control blanket. But long-term erosion control will be better if there are also native grasses present. I include below an excerpt from a previous Mr. Smarty Plants answer that gives advice on suitable grass species. The species recommended for North Carolina should also be suitable for Virginia.
For plant recommendations, let's start with grasses since the extensive fibrous roots of grasses are very effective in holding soil in place. Native warm weather grasses will come up from seed after the weather warms in the spring. The seeds can be planted now beneath erosion control cloth (see comments below) or you can wait until early Spring. Here are some species native to your area of North Carolina that will grow in clay:
Andropogon gerardii (Big bluestem)
Elymus canadensis (Canada wild rye)
Elymus virginicus (Virginia wildrye)
Eragrostis intermedia (Plains lovegrass)
Muhlenbergia emersleyi (Bullgrass)
Muhlenbergia schreberi (Nimblewill)
Panicum virgatum (Switchgrass)
Schizachyrium scoparium (Little bluestem)
Sorghastrum nutans (Indiangrass)
Tridens flavus (Purpletop tridens)
Tripsacum dactyloides (Eastern gamagrass)
Elymus canadensis and Schizachyrium scoparium will grow well in partially shaded areas, and in full shade I recommend Chasmanthium latifolium (Inland sea oats). You can choose wildflowers to go with the grasses from the Virginia Recommended list of commercially available native plant species suitable for planned landscapes.
Our National Suppliers Directory site for your area can be searched for nurseries and seed companies specializing in native plants.
To prevent erosion until the plants are established, use a biodegradable erosion control blanket such as the one available at Native American Seeds. Native American Seeds also sells a variety of grass seeds. Wildflower seedlings, including phlox, can make their way through such a blanket. If you choose to plant phlox, remember that some species, such as Phlox divaricata (Wild blue phlox), prefer partial shade, while some others, including Phlox drummondii ssp. drummondii (Annual phlox), do best in full sun.
There are several How to Articles under Large Scale Wildflower Planting (for instance, Meadow Gardening) that should have useful information for your project.
Below are photos of some of the plants from our Image Gallery: