What you need is to use "MYBB Merge System" you can find out more about it at this URL: http://www.mybb.com/downloads/merge-system however it looks like vBulletin 4.2 is not supported only vBulletin 3.6, 3.7, or 3.8.
You may pick up some online converter (like CMS2CMS) to perform the migration. This online tool doesn't require any programming knowledge to perform the migration. I'm not sure about vBulletin to bbPress migration, but you may contact them and ask.
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