If we check the other side, only after raising the Invoice, any Vendor discharge the output tax liability. I feel It would be erroneous, if any one take the Input Credit without a valid invoice for the Purchase . However this may create problem to the your Vendor. As you have a valid documents for taking VAT Input Credit, Credit cannot be denied. The Principle is that once it is eligible for Input Credit by the Provisions of the ACT/RULES, there is no need of one to one correlation of input credit with the output tax liability.