It's good that you took Marcel to the vet...dehydration happens so fast in kitties. You can tell if he is dehydrated by pinching the fur on the back of his neck...if it snaps back in place he is hydrated...if it stays humped up then he probably needs more fluids.

The thing that worries me is the fact that you have one inside cat and one outside cat. The inside cat has a tremendous chance of catching something from the outside cat if they have contact at all...vaccine or not. The first thing 2 cats do to greet each other is sniff noses...bingo, you have contact! If you can, I would try keeping them both inside only. I know that it is hard to see your kitties cry to go outside, but they will get over it eventually and you are being much kinder in the long run by protecting them from other animals, other humans, cars and disease.

By the way, welcome to PT!!