[final kaoss]

Well you didn't quite get that right. See you are using vbseo. That url structure will be lost if you switch to vbulletin 5 because there is no vbseo or dbseo for vbulletin 5. You would require someone to make you some custom rewrite rules to throw into your htaccess.

The furthest version that you can move up to while keeping that url structure intact is either the latest version of vbulletin 3 or the latest version of vbulletin 4 since both support dbseo and both can import those rewrite rules.

Also what I would do in your case is just keep vbulletin 3 installed but update it to the latest version and replace vbseo with dbseo because vbseo is a security risk...