Which VB version should I buy to integrate with Drupal 6?

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I'd like to integrate VB to my existing Drupal 6 site so that people who register on my site can directly access the VB forum.
I see that there is a vBSSO which does the trick but it apparently works only up to VB 4.2.
However I like the look and feel of new VB5 much better that VB 4.2, so prefer not to get another VB 4.2 forum.
So I'm wondering which version of VB shuld I buy to make sure that the integration goes smoothly?
Thanks

[Wayne Luke]

You'd have to ask the maker of the plugin you're going to use. You'll probably have to use 4.2.X at this time though.