The Additional Insured Vendors Endorsement, CG 20 15, is for the manufacturer or distributor that adds the retailer or other vendor to the general liability policy of the manufacturer or distributor as an additional insured. Your situation is the opposite of this.

If the manufacturer wants to be named as an additional insured on your insured's liability policy, you might try using the Additional Insured Designated Person or Organization Endorsement, CG 20 26. This endorsement is designed to provide protection to a person or organization whose status may not otherwise qualify for one of the other additional insured endorsements. Or, you might try a manuscript endorsement that will satisfy the intentions of the parties involved.