Can I post a vendor refund on Online quickbooks? If so, please share the exact posting I can do. Thank you in advance.

Vendor refund is akin to a credit memo; 

Debit Accounts Payable (contra posting) tagged to the supplier

Credit Expense that the refund is based upon

When you get the money, 

Debit Bank Account,

Credit accounts payable (you should be able to match the bank transaction to the memo above)