Is the IP shared in Plesk? How many IP's are dedicated to your VPS?
On our dedicated server, if an IP is dedicated to a site, for example, if the site has a secure cert and requires a dedicated IP, it indeed will serve up the page via ip address. You might see if you can go into your IP pool and at least temporarily dedicate the IP to the site. This, of course, will point any other sites on your VPS to that web site.
Might not even be possible with a VPS, I do not know, so test with caution.