Seems it could just be a matter of insensitive timing then and your vet not explaining the system they use. The vet could have billed you and left you to recoup your expenses from the company so you would be a direct claimant. By cashing the cheque you would legally be agreeing the payout. Because you are the claimant the vet can't agree the payout, only you can so they need your signature. So far they only have your vet's word that this happened. They need you to confirm it. Imagine the potential for fraudulent claims.

It must have been a huge shock when your cat had a reaction which would make anyone suspicious of such paperwork. It's not really any different from a third party insurance claim.

Every state has its own applicable laws, many times very different one from another. Your lawyer will determine what to do in your case.

I would take the positive side of the situation: 1) your vet immediately informed the manufacturer on your cat's adverse reaction (great vet, attentive and interested in proper responsible work); 2) your cat received the right treatment for it and she is doing fine now.

Now, the manufacturing company could not have refused or delayed the answer, only because they want to keep selling the product, I am sure that's why they have offered to pay for the inconvenience. If they chose to pay for the veterinary services (and not you, for the suffering caused), it's probably a propaganda issue, rather than a compensation for the side effect. The vet will sell the product to others, not you.

If you sign off the letter, it will help the manufacturer in his propaganda, and the veterinary institution (although they haven't asked for a money compensation, they only did the right thing, that is to report the case). In this picture, you and your cat are only a pawn necessary for the king's next move, so to say ...