For now, to keep the temperature as high (and constant) as possible, put the tank in a warm space, like high up in a cupboard. I would keep him in the tank, not the vase, as a the larger amount of water will keep a more stable temperature. Wrap the tank in towels and blankets to keep the warmth in.
Keep the sharp decor out of the tank and put in soft or smooth things - coffee mugs, PVC pipes, hamster tubes. It doesn't have to be plants for now, just things that he can hide in. Just make sure it's clean.
If you want to add salt to the water, use AQ salt. You don't need to, though. Heat will do far more than salt. Keep the water super clean. I'd suggest a 50% change every second day.