Firstly SQL 2008 is supported as you say. It most however be SQL 2008 SP1 64 bit as per compatibility guide and note the following to pints.
g. Enterprise Vault Reporting is supported, except for: Vault Store Usage Summary, Vault Store Usage by Archive,
and Vault Store Usage by Billing Account.
h. Requires Enterprise Vault 8.0 SP2 or later service pack.
With regards a direct upgrade path this really depends on SQL rather than Enterprise Vault. For example if you are running SQL 2005 32 bit I dont believe you can then directly upgrade to SQL 2008 SP1 64Bit. MS resources will answer your queries in regards upgrade paths.
In regards Enterprise Vault I would ensure that you have a good backup of EV Environment before undertaking this work, then depending on if you can perform an in place upgrade or not will determine the next steps. I.E. If you are moving to new hardware than the databases will need to be migrated and permissins set up for EV as required. If the server has a new name etc then changes will need to be made to Enterprise Vault to reflect this. The following technote should assist.
http://support.veritas.com/docs/273272
Hope this helps.