There's not exactly much to simplify at the moment is there?
Could you use Get-Mailbox I guess which would save you typing the username in the path. e.g.:
Get-Mailbox "" | Export-Mailbox -PSTFolderPath "C:\PSTFiles\$($_.SAMAccountName)"
Adding a VbScript wrapper would, in my opinion, only make it far far more complex than you need.
Chris