I got given a test date last year that was to be in the middle of a trip with my dad, that we'd planned and booked months in advance, to go and fulfill his lifelong dream of hiking the Inca trail in Peru. There was no way I was going to let him down only 2 weeks in advance (and at his age, you never know if something may happen before we could reschedule...). So following the instructions on the website I wrote to CIC explaining the situation, and they rescheduled it only a month later. The delay was totally worth it to see the look on my dad's face when we hiking
So I guess it depends on the vacation. Good luck!