Sounds like you need to take drastic measures here as a heavy infestation of buttercup means you have poor drainage and damp, heavy soil. Even if you get rid of all the buttercups they would just come back. The most practical solution is to cover the area with wet newspaper 5 to 6 sheet thick then add six inches of new topsoil or wood chips on top of the paper. The buttercups and all the vinca will die from lack of light but the new soil on top can host another try at growing vinca or some other groundcover like ajuga under your trees. Or you can just leave the wood chip mulch on top of the paper for a couple of years and then mix this organic matter into the soil after it has rotted. You are raising the level of the new planting bed so it drains quicker and discourages new buttercups. Add lime to the soil as well. Buttercups love acid soil and lime helps to sweeten the soil and discourage this weed. Keep growing, Marianne Binetti