Record the vocal and mix the song as you normally would. The steps necessary to set the vocal will happen at the end of the mix-down phase of the project.
Copy the vocal to one additional track. Chances are, the existing vocal is panned to the center. Pan it all the way to the left, and pan the copy track all the way to the right. At this point, you will hear no difference except possibly a change in volume. The vocal will still come from the center of the mix.
Add a delay effect to the copy track as an in-line effect. Zero all the parameters of the delay, then add a time delay of 25 milliseconds. You will hear the vocals thicken with an artificial double-track effect, but one that separates the panning. Experiment with the delay time. From 15 to 40 milliseconds will deliver separation without a slap delay effect.
Adjust the equalization, compression and reverb of each vocal track independently. Slight changes will further emphasize the differences, making it less likely that voice canceling phase inversion can occur. Complete your mix as you normally would.