Answer: Because of the plants' proximity to the street, I suspect that road salt has harmed your vinca. Other factors, such as heat reflecting from the pavement and air pollution, could also be contributing to the problem. You might try replacing the vinca with creeping juniper (Juniperus horizontalis), which has demonstrated some tolerance to salt and is evergreen as well as very low growing. This plant will also tolerate the hot and sunny growing conditions a bit better than vinca could.
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