Adding onto what TheMrDrProf said: basically LetsEncrypt just wants to know you actually control the domain you’re using to get the certificate. With HTTP challenges, your domain has to resolve to a working HTTP server. With DNS challenges, you need API access to your DNS provider so that Certbot can set a temporary record that proves ownership.
If you’re using NPM to manage your certs, then as TheMrDrProf said as long as the HTTP request from LetsEncrypt can make it to your NPM through the VPS proxy, you should be able to pass the challenge and get a certificate. The IP address of the domain doesn’t really matter as long as the request makes it all the way to the challenge HTTP server, which in this case is NPM.
In NPM, you should see “Use a DNS challenge” option. If you use that and your DNS Provider is supported (if not, I recommend Cloudflare), then your VPS proxy does not even need to be working in order to renew certificates. This has a few advantages such as being able to shut off unencrypted traffic on port 80 completely.