So if your looking for one thing to supplement your Van Ricthen's Guide to Ravenloft, you probably can't go wrong with the 2E Ravenloft Campaign setting. This combines material from the original black box AND the Forbidden Lore supplement IIRC. So your getting two separate things together in one package, which is nice. (This is otherwise known as the Red Box) Plus original Domain/Dark Lord lore/info that you may freely choose to use or ignore or supplement in your 5E Ravenloft games. Also you can get something called Dark Lords which is 96 pages of more Dark Lord goodness, such as Bluebeard, Headless Horseman, the House of Lament and Ankhtepot.
There is also, if your have a certain Death Knight appearing or taking a slight visit/detour to Sithicus, When Black Rose Blooms which is the Domain guide to Lord Soth and his domain.
If your doing anything Van Richten related or going hardcore on having him be a source of knowledge on various monters, there are also the various Van Richten's Guide to (insert thingie) such as Van Richten's Guide to the Liches or VRG on Ghosts, etc ,etc, etc. There is also Van Richten's Secret Arsenal which is where both the Weathermay-Foxgrove sisters appear. That gives a ton of info about them in that book. AND you get a picture of them.
Opinions may be mixed about it, but I would also probably suggest The Bleak House adventure module. You not only gain Domain info on Dominia, but also its Dark Lord I believe (ala Black Rose Blooms with Sithicus). Plus the letter that Van Richten, wrote on page 11 of the 5E guide, seems like the perfect thing to read to the party at the start or end of playing The Bleak House for a 5E game.
Masque of the Red Death is pretty cool as its basically "Ravenloft" but in a version of "Victorian England" in what is known as Grim Earth. There is a 5E version of this on DMguilds which is pretty cool and totally recommend checking it out. *Your mileage may very if you go with its class system take or you just sub dnd classes in there.
Also since Ravenloft is kinda/somewhat/I think/Vistani/detoured into the Shadowfell this edition, you also got the 4E Shadowfell: Gloomwrought and Beyond book for lore stuff as well. (including the bases for the 5E Vistanti.)
Those are some of my takes.