vBulletin already makes RSS for you.
AdminCP -> vBulletin Options -> External Data Provider -> Enable RSS Syndication
That allows vBulletin to create RSS feeds for your entire board, or for a particular thread, or even for user blogs if you have that addon.
As for your spam registration, sign up for
reCaptcha and set your Human Verification Manager (underneath vBulletin options on the left side of the admincp) to recaptcha. This is the most tried and true method of preventing spam registrants.