Avinash,
1. you cannot specify a particular upto date in app.
2. for point 2 - In APP, All company code parameters screen enter the special G/L indicators you wish paid for customers and/or vendors, no spaces between them.
3. if your vendor is also a customer, then in the master records(accounting section) check 'clrng with customer' in the vendor master and 'clrng with vendor' in the customer master record. Also give the appropriate account numbers, that is, the customer account number in the vendor master record and the vendor account number in the customer master record(in the control section).
hope this helps.
regards,
Aziz