15 members online browsing and posting, a smart shared hosting account will do.
I suspect it's an issue with the configuration of any of the VPS"s elements.
With that kind of traffic, cloud flare is useless in my opinion.
The name server change should be a temporary issue, potentially. But not consistent, as DNS simply updates for everybody.
Run the web site in debug mode, while it does add more queries to a page, the errors might give you more insight, hopefully.
Also check the VPS's web server error.log file (perhaps in /var/logs/apache/error.log) when these errors happen, might also say more about the error than XenForo spits out.
If they load the site and get a blank page with the error "couldn't reach the server, timed out" it might be a DNS issue, unrelated to xenforo.