I just posted a similar question recently on the ground cover forum because we have an area with similar conditions where we wanted a ground cover too. (We're in zone 6).

Just a few questions:

1. What hardiness zone are you in? It's unusual for Vinca minor to do poorly in general (normally it is a total thug) so am wondering about your zone.

2. When you say "solid" clay, do you mean there is so much clay the ground actually feels like a 3-week old biscuit, kind of hard and dry, and when you dig it, it comes up in big dry clumps (dry clay)?

Or is it more like wet gobs of clay, and the soil almost seems like you could make pottery out of it if you formed it into a shape and let it dry (wet clay)?