There are two possible solutions: migration or integration.
To migrate posts and users from a vBulletin forum to a Drupal forum, see:
- vBulletin to Drupal - Module for migration from vBulletin to Drupal.
- Advanced Forum - Module to build Drupal forums similar to systems like vBulletin.
- Choosing Drupal forum over vBulletin - Article on this option.
To integrate a vBulletin forum within a Drupal website, see: