Still I suggest to use standard process.
If you don't want to to avoid the price indicator date in vendor master or info record, then you can make the field as mandatory and tell all user to give the value 3- Current date for the field.
If you have existing vendor, then you can use mass changed for the field from XK99.
To change the field value from blank to 3 - Current Date.
If you want to do it default via badi, then you can use badi ME_PROCESS_OUT_CUST.
Consult with your ABAP team to make the field value as 3 - Current Date at the time of Contract creation.
