Moss lined pots are hard to keep watered well. Especially in indoor conditions. It would be ideal to move it to a regular, plastic hanging pot and change as much of the soil as possible to a mix of standard potting mix with about 1/3 perlite added. You could also just add more soil to the pot it's in, then when you go to water it, soak it in the sink for about an hour, drain good and hang back up. This might be a better option until the plant stabilizes. Once it stops dropping leaves or dieing back, you can put it in a regular pot...

And yes, being right above a vent could be contributing to the problem. I would move it away from the vent.

Denise in Omaha

I agree with Denise. Add more soil, give it a nice long soak, and move it away from the vent immediately. If it is rootbound and you are already struggling to keep it hydrated, the vent is probably the straw that broke the camel's back.

Then give it a nice plastic pot in the spring.

I don't think people should be terrified to repot a healthy Hoya in winter, if there is a good reason to do so, but I've had weak ones kick it from the shock. You just have to use your good judgement.