The best self-tanners give your skin the kind of golden, sun-kissed glow that usually only comes after days of laying in the sun. It doesn't mean, however, that the layer of color is keeping you from getting a real tan underneath — in fact, it doesn't protect or create any form of barrier on the skin. Though it looks real, that darker color forms from a reaction between an ingredient called dihydroxyacetone (DHA) and amino acids. This is different from exposing your skin to UV rays from the sun, which then causes melanin to spread to the surface in order to protect it from damage, thus creating the darker pigment in your skin.
That means that if you get a spray tan or use self-tanner before vacation, you can still tan (and burn) underneath it. You'll still need to wear (and frequently reapply) a good broad-spectrum sunscreen — especially since the deeper color could also disguise your sunburn, meaning you may not notice the red until it's too late. Don't let that faux glow fool you into thinking you need a lower SPF, either: your skin isn't really tanned. Always use SPF 30 or above when you're out in the sun.
If you're worried about what all those sun creams will do to your fake tan, opt for oil-free formulas that won't break down the color so quickly. There are some self-tanning products out there that contain an SPF, but those work best if you aren't planning on laying out in the sun all day. Save those formulas for at-home use, and choose a regular sunscreen, or maybe a gradual tanner with a built-in SPF like Jergens Natural Glow + Protect Daily Moisturizer Sunscreen SPF 20 ($5). This will enhance the glow you've got — but allow you to build up your own tan, too, so you'll have less risk of overdoing it.