Migrate from Drupal 6 Advanced Forum to Vanilla Forums Open Source

The website is based on Drupal 6. It uses only free as in freedom software. I have seen that there is a Drupal Module called Orchid that is meant to help setting up Vanilla Forum.
1) Is migrating From Advanced Forum to Vanilla Forum Open Source easy on Drupal 6? Would much of the content already on the Advanced Forum be lost?
2) Currently, many members do not view the Advanced Forum webpage Instead, they subscribe to a mailing list, which also puts their messages and replies into Advanced Forum. Would members be able to send posts to Vanilla Forum from a mailing list?
3) There is a terrible problem with spam on the Advanced Forum. Would Vanilla Forum Open Source be much better at blocking spam?
4) Is Vanilla Forum Open Source technically a 3rd party solution with a Drupal bridge?
4) What is the licence of Vanilla Forum Open Source? Is it GPL?
Thank you for your help! It looks like a great Forum to me