I've never thought to do it, but my guess is it's not going to prevent you from burning once you get there. I don't see the spray tan having any UV-A and UV-B protection. If I had to do it, I would go a few times to the tanning bed to get your skin a bit of color.
Spray tanning is for those who are never or rarely in the sun. I recommend you just get the high SPF sun block. Even with that, you'll get a little sun.
I'd say don't waste your money on spray tanning.
While on vacation, you should protect your skin and limit your time in the sun. Even on cloudy days and shade, there is still a good chance you'll get color.
I just got back from Jamaica in March and did just that. I laid in the shade alot and managed to get a tan. Alot of those days were cloudy, and you don't feel the sun on your skin, but you still are getting the sun. I did'nt tan before I left and used SPF 30 and gradually reduced the SPF as the week went by.
Have fun, the sun is strong down there.