Between the shade and tree roots in your yard, your new vinca plants need special care to get started. Once they are established, they will grow fine, though more slowly than you might hope. Special care means digging a hole for each plant slightly larger than the root ball  to eliminate (temporarly) competition from tree roots. In such situations as yours it almost helps to mound a bit of good soil on each spot where a plant is to go and set the new plant slightly above ground level, giving it a longer period without competion. Then it can fill the holewith roots, gather strength, makes leaves and stems, before going into battle below ground.

I hope this is clear, hopeful, and helpful. If I'm off the mark, or you can say more about your growing conditions and the size of the periwinkle plants that failed, please let me know.

P.S. there are other plant options if your goal is a lot of groundcover.

Video reply: how to grow healthy and thriving Periwinkle plants, aka Vinca; the difference between Vinca major and Vinca minor; instructions for soil preparation and tips for planting; shade vs. sun. 

Periwinkle is available in a range of colors. Recommended plants include Vinca minor 'Bowles' and Vinca minor 'Atropurpurea.'

This video reply was first broadcast online during our live plant and gardening Q&A show; the video you see here has been edited from a recording of that show. 

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