Seems to me that the quickest way to do this would be to repurpose the Yahoo/MSN/ICQ/Skype fields already in vBulletin.
I'm figuring that this would involve phrase edits (change Yahoo to Facebook, MSN to Twitter, etc), code edits to the MEMBERINFO and possibly POSTBIT (or POSTBIT_LEGACY) templates, then removing the existing contact information in the member's records (probably easiest through phpMyAdmin). After this, it's simply a matter of replacing the Yahoo/MSN/ICQ/Skype icons with Facebook, Twitter, etc icons (for displaying on the postbit).
Now, packaging all this into a one-click mod.... that would require a better coder than I am. I'm more of a "hands-on, see where I screwed up" kind of guy.