Chinoides has excellent sun tolerance so not sure that is a contributing factor. But they are getting munched on and I'd guess that to be root weevils as the leaf edge notching is characteristic.

Do they get watered through late summer and early fall? That is key for rhodies to set flower buds for the following spring.

They look nutrient deprived rather than diseased, so I'd make efforts to clear away all that vinca from a good distance around the base of the shrubs and apply some rhododendron fertilizer. That and adequate watering late season should help.

Is there somebody who will do a quick and dirty pH test for you? Cooperative Extension used to do it for $3, and I've known small garden centers to do it for free. I'd want to rule out something strange going on like someone mixed lime mortar in the area, or buried concrete, or even a limestone ridge.

You can do a simple pH test at home. Most nurseries and garden centers carry at least one brand (Rapitest) of a soil solution pH test that is remarkably accurate for the cost. As long as you use distilled water for the solution. That is critical as tap or well water can throw off the results.

Are the other nearby rhodos doing OK?