I would advise against it. I wouldn't take a puppy out in pubic until they are fully vaccinated. Remember you'll have to stop for potty breaks etc. so even if you don't pick something up from the show grounds.
There will be other shows - I'd wait.......
I agree 100% with Lilie. I spent many years at horse shows, trained and showed horses for my veterinarian, who also bred and raised very good Arabians. As far as puppies go, he was always adamant about keeping them home until they are fully vaccinated. Also, a puppy at a horse show? Ummm, not a good idea. Too much going on, too many people rushing to get their horses ready for a class, too many chances for a puppy to get underfoot, and you don't want your puppy underHOOF. If it's not a big show, then is it that important? Is it a breed-sanctioned show where you've got a good chance at a halter or performance championship? If it is important to you, then wait to get your puppy. However, you put parentheses around the word 'breeder'--does that mean you question whether you should leave your puppy with that breeder any longer? Can you get the puppy and have someone sit with it while you're gone?
These are just some things to consider.