Yellow leaves are usually a sign of 1 of 2 things--overwatering or lack of fertilizer. If you put them in 2 weeks ago and you've watered a few times that is probably too much water.

What size pot do you have them in? If it's a large pot with very little sun to dry them out then you shouldn't have to water more than every couple of weeks. Of course this depends on how hot it is. In the height of summer you may have to water more. Right now let the pot dry out completely and then water. The worst thing you can do to a plant is keep it wet all the time--it is sure death, from root rot and disease.

When you let the soil get dry at least half way down the pot, before you water, the plant makes more roots that go deeper into the soil in search of water and that's what you want. A good root system makes a good plant

oilpainter; The Vincas are not in pots, they are in a flower garden. I will lessen the watering....Thanks...

These are heat lovers. It may still be too cold for them.

What kind of Vinca? This is one of the most confusing common names, of ALL! Are they 'real' Vinca( either Vinca minor or Vinca major) or are they Catharanthus roseus, which is often called 'vinca', but isn't?

Cultural care is quite different for the real Vinca species than the Catharanthus.