Bacula includes solid VSS support since version 2.0. This should allow you to safely back up a consistent snapshot of an Exchange database. But bacula does not involve the VSS writers in any sophisticated way, so even if you run a full backup, this is not registered by Exchange, the last full backup timestamp is not set and the transaction-logs are not flushed. This is the way it is built and not a bug.
Note that the VSS Writer for Exchange appears to be disabled by default in some versions of Windows Server, including Small Business Server 2003. You may verify this is the case by examining the VSS messages, which will look like this:
27-Mar 03:45 exchange-fd: VSS Writer (BackupComplete): "System Writer", State: 0x1 (VSS_WS_STABLE) 27-Mar 03:45 exchange-fd: VSS Writer (BackupComplete): "FRS Writer", State: 0x1 (VSS_WS_STABLE) 27-Mar 03:45 exchange-fd: VSS Writer (BackupComplete): "MSDEWriter", State: 0x1 (VSS_WS_STABLE)
Note the lack of an Exchange Writer entry. This Microsoft knowledge base entry discusses how to enable it, which will allow you to use 3rd party backup packages such as Bacula.
You may also find the ExMerge program useful. It is a Microsoft application that allows you to extract and merge individual mailboxes as PST files from and to an Exchange server. It should allow you to extract all of the individual mailboxes into seperate PST files, which would then be backed up by bacula.
The existing bacula Exchange plugin uses the esebcli2.dll api, which is not supported under Exchange 2010. Exchange 2010 can only use VSS to do backups.