Hi...
set the default value using the Exit or BADI ask u r ABAPer in this regard
1, Create a vendor in your Account group as the reference vendor account
2, By t-code:XK01, create new vendor with reference vendor of step 1,
or
Transaction code for creating Transaction variants is 'SHD0' go to this Transaction and give your Transaction code in initial screen and press create button it will take you into Vendor master transaction. just give the Standard variant and description and save it.
you will get exactly what you want.
Regards