How can take VAT set off

Hi,

One of our client is in business of Erection of power plants.

They purchase a material which is required for their works and pay VAT on that items.

They sub-contract their work to the small contractor and get the works from them.

They raise sales ivoice to their customer.

How they can get credit of VAT paid at the time of purchase.

Can they set off against service tax or set off against WCT?

rgds

dhananjay