Hi K F,
The vaccination protocol depends on the diseases focused, the age of your horse (foal or adult horse or mare..), and the competitions laws.
If your horse is up-to-date for vaccinations, my advice is to vaccinate him at least one week before a show. I vaccinate a horse if and only if he is in good state. Indeed, if the horse seems sick or in immunity depression, it’s better to post pone the vaccination and treat the horse for the disease or help him with dietary supplement (see Or spiruline on or-vet site for example)