Vendor will not supply copy of contract?

Yes they are suppose to provide a copy of any agreement or contract you have with them unless it was authorized as verbal. Tell the State Attorneys office in you city or state of your situation immediately. Tell the vendor you will; stop any monies and put on hold all outstanding checks to them until matter is cleared up. when they reply as they will that a copy is being sent to you, tell them you want it hand delivered and notarized as the legitimate copy of said contract you agreed to and hand deliver it.

Yes they are legally required to provide you with a copy of the contract (how come you don't have a copy already?)

If they continue to refuse, send a registered letter asking for a copy of the contract to be sent to you within 14 working days, otherwise you will file a civil suit to demand they turn over their records.

Did you loose your copy of the contract?

What you need to do is tell them in writing that if you do not receive a copy of the contract in 10 days that you will consider the contract null and void and go find another vendor.

Read YOUR copy. What, you signed a contract and did not keep a copy? Even if they supply you with a copy, how do you know it is the same as what you signed? The ONLY way you can compel them to supply a copy is REFUSE to comply and let them SUE you. They will then need to produce the contract as proof you are in breach.

Why doesn't your company have a copy of any contract that they signed? Relying on the vendor to supply a copy is dangerous.

You should have received a copy of it when you signed it. They're not required to provide you with additional copies.