Are you sold on switching to vB? vB is a step down in my opinion. There are tons of forums moving away from vBulletin, they've really gone downhill the last few years. I've been using phpBB since 2001 and it's come so far, 3.1 is so much superior to 3.0. If you're going to switch, go to XenForo, although I don't think it's worth the effort. Just find a coder to make a new extension to make your old mods work under 3.1.
What exact features do you want from phpbb-seo? Are we talking meta keywords tag, people friendly URLs, etc? The days of SEO'ing the heck out of stuff like forums has passed in my opinion. People friendly URLs don't matter to search engines. Meta keywords are ignored by search engines. Meta title and description is still useful although Google will ignore them too at times.
phpBB 3.1 has eliminated thousands of duplicate URLs on my forums. Gone are the view=next and view=previous URLs for search bots. Gone are the forum ID in the URL for search bots, etc. They've updated the <title> tag to be more people and SE friendly, etc. 3.1 is a
massive step up and honestly the reason phpbb-seo is not compatible is probably because it's made obsolete by all the 3.1 core upgrades.
I've been using 3.1 for about a year and a lot of extensions (formerly mods) have been developed. The phpBB 3.1 upgrade will be a lot easier than switching to a new forum software so you should really think about what you want out of it before you make a drastic change.
[edited by: Swanny007 at 8:12 pm (utc) on Mar 21, 2016]