Actually being, perhaps, too patient, I would amend the soil in the sparse areas, depending on the invasive tendencies of the Vinca to move in. If the soil was suitable the Vinca would already be there, since it's design is to cover our entire planet.

I have amended those sparse areas with an inch or 2 of leaf compost from the municipality where I live and loosened the soil with a digging bar and pick-ax. Do you think it would be a good idea to spread some nitrogen rich fertilizer to promote growth to these areas ?

Knowing how nothing stops it here, it is a mystery to me.
I, generally, do not try to force growth with high Nitrogen and I use slow-release sorts.