Answer: Vinca minor can take several years to fill in a given area if planted on the typical 18 inch centers, so erosion is a serious concern. You will also need to kill o9ff any existing grass or plantings, amend and prepare the soil and water the vinca until they become established. You might try planting a horizontal band at a time starting at the top and working your way gradually down the hill, and plant the starts closer together. This would avoid exposing large areas to erosion. You should mulch in between the plants using a large size of mulch (such as bark nuggets) which is less likely to wash down hill.
You might also consider having the hill regraded since you are already considering replanting it. This could eliminate or greatly reduce the erosion, as well as make it much easier to establish the new planting. Since that hill is so steep, I would strongly suggest you consult with a landscape architect or an engineer with formal training in drainage and grading with regard to the grading issue. Sometimes, depending on the drainage patterns involved, a series of terraces or retaining walls are recommended rather than maintaining such a steep slope.
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