The instructions for setting up custom domains in Website Settings says: “To enable a custom domain, add it using the form below, and then configure a new CNAME record with your DNS provider to point it at treshenry-net.vapid.site .”
The problem is that CNAME records are meant to handle cases like resolving the A record for mydomain.com when www.mydomain.com is requested. CNAME records can’t be used for the Vapid hosting use case because CNAME records are not allowed to exist for the root domain (according to spec). I figured this out when trying to configure DNS with Joker for my domain name to resolve my Vapid hosted site.
For more detail here’s a good article (as I know basically nothing about DNS): https://medium.freecodecamp.org/why-cant-a-domain-s-root-be-a-cname-8cbab38e5f5c
I’m experimenting right now with Joker’s “URL forwarding” feature to see if I can get that to work for this case. I’ll update this topic with my results.