On other forums too?
What OS/Browser?
Could possibly be a misbehaving advertisement, especially if the forums all use the same ad network. Other than that:
If you're on windows - could be malware or a bad extension somehow detecting vBulletin on page requests and injecting bad links.
If you're on mac - could be a bad extension (if your on FF or chrome, for instance). I've seen those do some crazy things. For instance, a relative once had a FF extension get autoinstalled that switched all google search results to links to random scammy sites.
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TLDR: Try disabling any extra extensions on your browser, and see if it still occurs. Also, try another browser. If it still happens - then it's more of a OS issue.