I was just wondering here, I have a very well established vBulletin forum and I am unsure weather it would be a good idea to try some SEO mods at a time when it is doing so well.

What do you suggest?

It can't hurt ;)
if you are using the default url setup with vBulletin, then you have loads of duplicate content

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I can say from experience that SEO indeed helps. 88% of my visitors come from google. SEO won't work overnight after you do the tweaks/changes, but gradually you will notice it working.

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tehPARADOX.COM

Definitely would help but my advice would be to be patient initially. When we tried vbseo on our board(an established one on default install) we actually had a decrease in traffic initially which picked up over the weeks.

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