I know this user is deactivated but thought I would respond anyway with some thoughts for anyone asking the same question.
Personally I don't know of an available tool for migrating vBulletin to Jive, perhaps others do. But I do know that Jive has APIs for inserting content and vBulletin also has APIs for retrieving content, users, attachments etc.
If you could programmatically work out how to create a user, then impersonate that user to pull and push their content into the new Jive forums you should be in business.
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Actually, JCS Consulting has done this for a few customers. We have the REST Client built so this is very achievable. Please let me know if this is something you are interested in.