Without seeing the agreement, I could not say for certain, but most likely this is a valid agreement. You contracted with X company to be a vendor at X's event, paying X $800 to do so. When you backed out, that was a breach on your part. X can sue you for the "benefit of the bargain" (what they would have received had you not backed out), which in this case is $800. Whether or not there is a refund is up to the contracting parties, not state law. If the contract
says no refunds, but only credit, then that is what is going to control. Now they DO have to attempt to mitigate their damages, meaning that if there are a limited number of vendors allowed, then they have to attempt to find another vendor (rather than just sitting around, thinking that they can sue you for the entire amount). Now if there are an unlimited number of vendor spots available (in that it is not artificially controlled), then you'll probably have no defense, whatsoever, to this breach of contract action.
Think about it: if they had breached, wouldn't you want to sue for the potential lost profits, as well as the $800 back, as well as the BBQ that you had already cooked?
Now bear in mind that you will not be punished for this breach of contract, but you likely are going to be on the hook for the $800 that you promised X company. You may also be liable for attorney's fees under Florida law. You should rather just take the credit and do another event with this company. That's your best option, because you really do not have a valid defense to their action.
I know that this is probably not what you want to hear, but it's the law. I hope that it clears things up anyway. Good luck to you!