Sir,
You need not worry whether the vendor is registered with Service Tax Department or not. Section 65 B (54) defines "works contract ' as-
"works contract" means a contract wherein transfer of property in goods involved in the execution of such contract is leviable to tax as sale of goods and such contract is for the purpose of carrying out construction, erection, commissioning, installation, completion, fitting out, repair, maintenance, renovation, alteration of any moveable or immovable property or for carrying out any other similar activity or a part thereof in relation to such property;
Since the vendor supplies the material and fixing the same it is works contract. therefore you have to pay 50% of 40% of bill amount charged by your vendor and pay it to the government. It is for the Department to collect vendor's portion of service tax from the vendor. You need not worry about it. To safeguard your interest, in my opinion, it is better you pay your portion of service tax.