perrera,
You have a few options here.
1) You could use the import method that you suggested. It should work though I'm not sure since I never have used YaBB. Be sure to backup your MySQL Database.
2) Waiting for a YaBB import script. It is only a matter of time before someone needs one
3) Last but not least, Try contacting vBulletin support at
support@vbulletin.com They may or may not write up a script to help you import but they will definitely will try to help get your vBulletin up and running