questions

1) Other than rewriting urls what does vbSEO acctually do?
-Will it dynamically add <meta> tags to threads, forums and posts?

2) Will vbSEO rewrite existing links in posts that already exist in the database?

3) Will vbSEO unrewrite urls in posts if it is uninstalled?

4) How easy is it to adapt the rewriting/seo to work for custom addons eg an article system?

5) are there any admincp demo's/screenshots/documentation for features and installation?

1) see for an overview: vBulletin SEO: The vBSEO Solution
2) yes, vbseo runs real-time on the fly. Nothing actually gets 'changed' in the database or files. It re-writes as the page is requested.
3) yes, same concept. since it never changes anything, if you disable it, it will simply return to default vb actions
4) It is very easy to work with, using CRR's (custom rewrite rules) so long as vbseo is in scope. Ie, vbseo works in the vbulletin directory and above it. If your board is in /forums and your articles are in /articles, it will be a little trickier to work with than if your board is in root and your articles are in /articles.
example CRR's : Custom Rewrite Rules for FAQ and Blogs - VBSEO Wiki
5) there is a full demo to play with: https://www.vbseo.com/demo/

2+3 present a problem to me, If a member copies a link from their browser windows and pastes it into a post, it will remain as written; if i then change the way urls are rewritten or remove vbSEO that posted link will become broken.

That's correct. You shouldn't change your URL format once you choose one though. That is a "set it and forget it" setting.

Does the admin cp for vb seo integrate with the vbulletin cp or is it seperate?

Also, i'm still trying to see what it is i'm getting for $150. Aside from mod rewriting urls (which I admit is a fairly large task) there dosn't seem to be much else done. (Adding rel="nofollow" to active links dosnt really count as anyone with any sense has done that already.)

Edit: in fact it seems (having just looked on my dev board) stock vb does that now anyway.

The vBSEO control panel has advanced options that do not fit into the framework of the vBulletin AdminCP Options area, so it is a separate interface. (It is linked from the AdminCP, however.)

have you tried the demo? There's 200+ options to play with.

https://www.vbseo.com/demo/

Do you have any other questions we can help you with Carnage?