Answer: Unfortunately, this is a very difficult area to plant. A slope is going to be relatively dry and the shallow maple roots will be very aggressive at taking moisture (and nutrients) out of the soil as well as crowding the roots of anything planted beeath the tree. So planting beneath this tree is a real challenge.
Junipers do best in full sun all day long, or in a minimum of six hours including the hour of noon. I am afraid this site is too shady so they will grow slowly and look very sparse and may dwindle away. Add the shade with the root competition and I would not recommend trying the junipers there.
You might be able to grow Vinca minor as an evergreen groundcover there, but it will take work to help it become established. You will need to dig individual planting holes for the starter plants, keep the area weeded and mulched, and water the vinca so the soil stays evenly moist until it becomes established. In spots where the tree roots are particularly dense, the groundcover may never be able to take hold and grow well despite your efforts.
Your local county extension and/or professionally trained nursery staff may have other suggestions based on a more detailed understanding of the planting site and your design goals. But, planting under a mature maple is notoriously difficult, so you may want to consider just mulching the area instead of trying to plant it. I wish I could be more encouraging.
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