Do a search on this site for 'lasagna bed'. It will cure what ails you.

Do the roundup now, then if more weeds germinate, do it again. After weeds are dead, rototill organic material into soil. Peat Moss is worthless, find some manure, a year or more older. Vinca loves it. Another option is it is available in your area is mushroom ompost. That is the composted growing medium from commercial mushroom growers. If this is done in the fall, in spring you are ready to go as soon as the soil is workable.

I'd suggest covering the area with black plastic now. The combination of lack of light and water plus heat generated by the sun should kill the weeds. Anything besides adding toxic chemicals to the environment.

I'll second the lasagna bed suggestion. It will give you an area to put your raked leaves in the fall and reward you with good soil come spring. Vinca's not a fussy plant, but giving it a good bed is sure to boost it's growth and help it establish into a healthy carpet for you.

I have it on my backyard hill and love it, especially when it blooms.