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is your VPS managed or unmanaged? if managed your host should be able to assist you further with this issue.also you should upgrade to SMF 2.0.15 for security.
Check the IP adress range: I had something like that and it turned out to be all coming from China. Since my forum does not target Chinese visitors I ended up blocking all Chinese visitors via htaccess. Problem solved
if your server software is not updated that would be a major security risk depending on how outdated it is. that should be priority to get adjusted. depending on your traffic I bet you don't need a VPS.
depending on your traffic I bet you don't need a VPS.