Get a moisture meter and use it.

Takes the guesswork out of watering.

These plants do need regular watering, especially if you have them in a planter of some sort. Anything in a planter will be more susceptible to heat and/or water stress.
What kind of sun do they get? Morning? Afternoon? Gazania are more sun-loving than vinca. I'm in zone 8, we give vinca afternoon shade when it's in the ground.
Neither of these plants are particular xeric (in my area) with vinca the lesser of the two. Personally I wouldn't plant these two together.
This is also a tough time of the year to get planters established.
Using a self-watering planter? You probably would benefit from a moisture meter.
Hang in there, the plants may bounce out of their slump.

Fall is the planting season for CA, not summer. Vinca will survive a planter with lots of water, but not gazania. The latter will do much better in the ground.

I want to thank you all for your comments. I do have a moisture meter. The problem is, when you have new transplants, it is not that useful as the roots haven't adapted to their new home.

In Phoenix gazanias, vincas, and portulaca are planted anytime including the hottest part of the summer as they are extremely heat tolerant and sun loving. I'm just wondering which requires more water. Gazania?

I know fall is the planting time for veggies, and more temperate-climate flowers, but certainly there are plenty of things to grow in the heat of summer here and I just got to experiment I suppose.

Thanks again to all! (three of the vincas died and the other three did fine...go figure)

Z

Vincas can be a little tough to start in the midst of summer--definitely make sure they have afternoon shade, and water more frequently until they are established. Also make sure you are mulching them--how big are the planters? While they can withstand heat, if the planters are too small you could be cooking the roots.

"Gazania, vinca and portulaca are planted anytime...sun loving."
"Sun loving"?
Hmmm, I'm wondering if we're talking about the same plant. I can think of 2 very different plants that are called "vinca" and a third that has the name randomly applied to it. Do you have a scientific name for the one you have or could give us a description please?
Just interested...

Sorry if I wasn't clear, I've been talking about summer vinca (catharanthus) as they call it and I guess it's not a true vinca. It is used as bedding plants in the hot desert southwest.

I planted six in total and the three on one side died and the other three are flourishing in 115 weather! It's quite interesting. Thanks all.

Z