Interleave won't stay stereo when exporting audio

When you exported, did you make sure "stereo" was selected in your export options? Did you have effects on the vocal? If you had no effects on it, it's going to sound mono even with stereo interleave enabled while using "stereo" export preferences. Mono in, mono out unless you processed the track with effects. In X1, it doesn't matter if you use stereo interleave or not. As soon as you put stereo effects on the track, the mono vocal will take on a stereo imaging sound. At this point, enabling or disabling stereo/mono interleave will not make a difference other than a volume boost. But you MUST have your options when you export, set to stereo if you want to hear the vocal with stereo effects.