You can use the formatted string format for entries in the file-table, i.e. [#filekey] to get the full path of the file. You should then be able to either set a property / CustomActionData-property and read it within a VBSScript. Please note also the following (extract from the linked page):
The value of [#filekey] remains blank and is not replaced by a path until the installer runs the CostInitialize action, FileCost action, and CostFinalize action.