I appreciate your concerns. Vetch is an annual plant. This means that if you are waiting until it's dry, it has already set and probably dropped the seeds for next year's crop. You need to cut ;it down when it forms buds for blooms to avoid seed set. This may be a tedious job, but it is the only way to keep the vinca and stop the vetch.     Vinca or periwinkle, by the way, is now removed from legal sales at nurseries because of potential fire danger created by the build up of dry leaves. You  might want to check with your fire department to see if they offer burn help. My understanding is that they will come and burn off dried plots if they are a  potential fire hazard.  Good luck with solving the problem.

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