Stephane
I don't think leaving the original PST is a valid option in Enterprise Vault...
Sorry!
Have a look on those articles, which should answer many of your questions:
https://www-secure.symantec.com/connect/articles/white-paper-v80-pst-migration-part-1-solution-overview
https://www-secure.symantec.com/connect/articles/white-paper-v80-pst-migration-part-2-tools-depth
https://www-secure.symantec.com/connect/articles/white-paper-v80-pst-migration-part-3-planning-tech-tips-and-best-practice
Cheers
Michel