Yeah, acme-client cannot use the DNS protocol like Certbot but acme.sh has a superb shell script where all you need to do is:
export VULTR_API_KEY="<Your API key>"
./acme.sh --issue --dns dns_vultr -d example.com -d www.example.com
The VULTR_API_KEY will be saved in ~/.acme.sh/account.conf and will be reused when needed.
So when setting up an email server using OpenSMTPD, which is super secure, as it's part of the OpenBSD project, you'll need to setup DKIM records in your DNS zone. However, every 90 days you need to get a new cert you need to change those DKIM records in DNS, so that is uber handy for the script to do it for you.
I'm going to write an article about how to do it on Vultr since they're offering $600 per article written if accepted. It's only $6/mo for OpenBSD 7.3 on a premium AMD EPYC CPU, 1GB Memory, 2TB bandwidth, 25GB with instant one-time snapshot backup feature too, for pennies.
If anyone is interested I can give you $100 credit after you pay for $10 of hosting. At that price that would be a free year of OpenBSD 7.3 "High Compute (AMD), Cloud Compute" lowest level boots up and starts serving my website and email in like 5 seconds. My page in OpenBSD httpd servers in 497ms, you can't even see it load. Instant emails over IMAP using dovecot (see article below).
$100 account credit, referral.
Here is the article I used to setup my email server, it works great! Although it was written long ago, you'll need to read my comment on the article posted yesterday on the fix for 7.3 and Rspamd 3.4 in a hosted environment.
I just started with BSD so I'm in no way even really good at it, but I can help you with it since I've must've set it up like 5 times to get it perfect! I also worked in the computer industry all my life, including at many NOCs, and I have great documentation!