How to balance vat and service tax

One of my client given sale and service to Indian Defence Service, they sale electronics goods and also provide digging and trenching service to Defence. The digging and trenching done by petty contractor and my client deduct tds from petty contractor.  My Client only registered with Delhi Vat Department.  In sales tax return they are not mention any where about digging and trenching service. Now Indian Defence asking for registration with Service Tax. So my question is in which category we should register with Service Tax and what will be service tax rate on digging and trenching service. (12.36% or work contract rate) Please help me. Thanks.