My husband has vascular dementia and I can’t leave him alone. I would like to know the best thing to get so I can hear him.

I think I would start with an inexpensive baby monitor. If you need more than that there are higher end monitors that have a camera option so you can see as well as hear, there are also various motion sensors and alarmed floor mats that will let you know if he is straying out of bounds. AgingCare has links to a few products

https://www.agingcare.com/products/1150/security-safety-falls