Not possible to set-off CST paid against VAT payable. Why?
CST is not VATable. in other words, CST paid on purchases are not allowed to be set off against the VAT credit on sales. This is so because CST paid on purchases is retained by the State (say, Haryana) from where goods are purchased. So when the other State (Delhi) which is charging VAT on sale will be at loss of revenue (i.e. tax revenue) if it allows the set off of CST paid on purchases (to Haryana).
Therefore, only local purchases (i.e. intra-State purchases) are allowed to take credit for the purposes of VAT payable.
Just sharing some details in brief.