I've never heard of problems with dyed mulch, but since I don't use it, I don't know much about it. It's doubtful the dye is to blame though. Perhaps the area became very wet and the mulch helped keep the soil too moist for your plants. Vinca likes quick drainage so if your soil has a lot of clay in it, maybe the mulch made the area too wet. I just don't know on this one. Sorry.