set fileName to "whatever_VAULT_20081203_SO.fcp"
set beforeVault to item 1 of my asTID("VAULT", fileName)
set extName to item 2 of my asTID(".", fileName)
set newName to beforeVault & "VAULT" & "." & extName
on asTID(delimiter, textHere)
set OLDtid to AppleScript's text item delimiters
set AppleScript's text item delimiters to delimiter
set theItems to text items of textHere
set AppleScript's text item delimiters to OLDtid
return theItems
end asTID
or
set fileName to "whatever_VAULT_20081203_SO.fcp"
set newName to do shell script "echo " & quoted form of fileName & " | /usr/bin/ruby -e \"puts STDIN.read.scan(/(.*VAULT).*(\\..*)/).join('')\""