Yes you can just cut and replant--but it is easier to serpentine layer them. This means that you weave the long vines in and out of the soil. When part of the stem is buried under the soil vinca will root in almost no time at all! I would probably dig up your vinca that is close together and move your plants so they are equally distributed then serpentine layer the vines as well--this will give you full cover in almost no time at all. By the end of the season you will be pleased--by next year you will be amazed!

Ryan