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Hand-weeding failed for us, even after fighting for several years on a regular basis in an established patch of vinca; the root mass of the vinca and the perennial grasses were simply too intertwined to get at and remove the grasses' root mass.
If the patches are spotty, you might try digging those spots out, ammending, and replanting with new vinca.
I doubt you'll find an herbicide that will kill the grasses without whacking the vinca too.
Good Luck!
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