CST paid cannot be set off against VAT payable. Whereas VAT paid (ITC) can be set off against CST Payable. The logic is CST is collected by the state where the movement of goods commences.
For Example if you are a dealer in Tamil Nadu bought a goods from Delhi. Then CST shall be collected by the Delhi Govt. So we cant claim the same against the VAT payable to Tamilnadu Government.
Likewise if you made sale to a dealer in Delhi, then CST shall be payable to the Tamil Nadu Government. Hence you can adjust the VAT Input Tax Credit available against the CST payable to Tamil Nadu Government.