Final answer:
Before the vacation could begin, the travelers had to fix a flat tire, which was the cause of their delayed departure.
Step-by-step explanation:
The event that occurred before you could leave on your vacation, according to the passage provided, is that you had a flat tire. This unforeseen incident delayed your departure, adding to the anxiety and anticipation already associated with starting your journey. The other options listed, such as running out of gas, forgetting passports, or changing your mind about going on vacation, are not mentioned as events that took place before the vacation started.