I've used vbseo on a few of sites.
Bear in mind that it uses mod_rewrite which can be very server intensive. The cost of using vbseo is not really the initial purchase but the upgrade you'll need to do to your server. With 200 or so users online an entry level dedicated server will start to struggle and combine this with the extra spiders (particularly slurp) who are taking pages more quickly becuase of the non .php urls and the problems get worse.
I believe them when they say that the new URLs probably result in higher rankings but you need to weigh this against needing to upgrade your server.
For a smaller site, I would recommend using vbseo as the load increase will be minimal and the higher rankings will allow your forum to grow more quickly.
For sites with lots of users online (200 or so) and lots of posts (meaning more spiders) you'll need to weigh up how much you need increased rankings and whether you'll need a new server.
Chris
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