A vendor provided our company with a letter on company stationery stating "if after 12 months you are not satisfied you may return the products".

There is no "statute of limitations" that affects the letter standing alone, since the letter is not a contract. The legal issue here is that you have, subsequent to the provision of the letter to you, entered into one or more complete contracts with this vendor. Those contracts may or may not incorporate by reference the terms of the 2007 letter, and they may by their terms expressly exclude any terms not incorporated in the subsequent contract. In other words, these post-2007 may provide -- and indeed, customarily would provide -- that all prior bets are off. If you can prove that the post-2007 contracts incorporated by reference the 2007 letter, you win. You may have to sue to prove it. If the vendor can prove that its post-2007 contracts expressly excluded all terms not expressly set forth in those documents, it wins. There may be facts on which you can base your claim which you have not mentioned. Those should be discussed with an attorney.