I'm so sorry ... I understand and I feel pissed for what happened to you.
I'd contact Certegy and find out WHY they denied the check. If it's BS, then I'd seek legal counsel.
After that, go lawyer shopping. And find out how the company gets its $$, and then start from there. Barristers these days are oft-fond of advertizing the "free consultation."
The vet may most likely be the ones responsible 'cuz they signed the contract with Certegy, not you. And legally you should have the right to a free copy of their file on you.
I would spin this into money and see if you can get hired by a local bank to sell Check 21 devices - they convert checks into Electronic Funds Transfers. Perhaps you can package & sell your story as a warning to businesses - Certegy is nothing but a liability and using them can lead to lawsuits.
Banks selling the new Check 21 device have been advertising that businesses get the $$ instantly - but your story is particularly gut wrnching enough that these vets owe you some danegeld if they want to avoid bad press. Hard times are here, and they most likely are hurting.
Screw the vet. They're the ones who commissioned Certegy to tell them yea or nay. If you can have the bank prove to you that there were sufficient funds in your account at the time of the transaction, then I would package the story as a media hot-seller. And with all the people being messed over by Certegy these days, your story has potential.
"If it bleeds, it leads." - journalism motto
You could pay off a local journalism student/class to write your story in media-ready form, print it up, and tape them onto mailbox posts (make sure you have a web address with all relevant pictures/evidence) and
HAVE LEGAL COUNSEL PREVIEW & AUTHORIZE THE FLYER TO PREVENT ANY LAWSUITS!
You can format the story to be a "You Be The Judge" kind of thing. Just present the facts, let it out, and see what folks do :)
But be nice and let the vet see it first - they may end up settling with you to get you off their backs.
Take what you can.
And Best Regards!