vAPP does not allow the following characters in the external DB password:
# at the beginning
The rest of the constraints, if any, are imposed are by the DB.
Also, note that you must apply the latest patches of 14.2/14.3 which fix an issue where some special characters were not allowed in the DB password.
For 14.2 apply vAPP CP05 or higher, for 14.3 apply vAPP CP01 or higher.