Here are my suggestions based on what I've read. I don't have much experience with ceramic coatings however.
On the Subaru, I would suggest the strip wash as others have said. I wouldn't polish vinyl. You can use Hand Revive Polish if needed. If the ceramic on the old vinyl is still effective, I would try using Ceramic Spray Coating on the new vinyl. Note, you might see a difference (sheen, dark vs light, etc) between panels based on the different ceramic coatings are used. Once complete you can definitely maintain Adams CSC with Ceramic Boost. I would try CB on the old coating as well but be aware that it may not cooperate. CB is designed to work with Adam's ceramic products so it may react differently with other brand's ceramic coatings.
For the Lexus, same as above. I would use the suggest maintenance product if you have it. You can try CB to see how it works. If you don't see streaking or ill effects then it should be fine. If you do see issues when using CB, you can go over the car with Revive Hand Polish to remove the CB and then go back to the installer suggested product. I probably wouldn't put CSC over a professionally installed ceramic coating. It would be a waste of product if the coating is designed to have a long lifespan. You should be able to get away with maintenance washes and claying as necessary.