It's hard to see the damage to the stems clearly, but they could have been chewed by cutworms or slugs/snails, or been infected by a fungal disease that destroyed stem tissues carrying moisture up into the foliage. Root rot is also a possibility, as Flowering Vinca is fairly intolerant of over-watering or poor drainage. The mulch placed so close to the stems might be making the plants more vulnerable to infection or the pests above.

To try to identify the culprit, you can look at the remaining plants at night to see if you can spot any active feeding, since both cutworms and slugs/snails tend to hide by day and won't remain on the plant. If the roots are still healthy, the plants might regrow if all of the damaged top growth is trimmed off, though they might be too weakened to recover.

If root rot is present, the plants cannot be treated or recover, and instead would need to be replaced. One way to verify root rot is present is to inspect the roots of a removed plant. If mushy, brown instead of white, and easy to break, then root rot is likely. (When pulled without much force, roots dead from infection tend to separate into a mushy outer layer and a stringy inner core, though on very fine roots this can be hard to see. Roots dead from other causes, or which are still alive, will tend to cleanly snap in two and take more force to break.)

Miri