Is this a constant for the vocalist, or is this a symptom of red light fever? It may be that the problem is less of a technical one, and more a matter of getting the vibe in the room right before tracking the vocal. Is the vocalist warming up before the take? Is he or she nervous? Sometimes singing in the dark can help, sometimes inspirational items in the room can help. Try to approach the problem from the tracking side before moving to the fix it in the mix side. If it's the best they're gonna be able to do, then compression and double tracking are good smoothers. Start with three takes of the lead vox, and comp the best sections to a fourth track. Then have them listen to the comp'd track a dozen times, singing along with it after the first half dozen listens, then have them sing the dub track in sync with the comp'd lead vox.
Mix the dub vox way down, and edit any parts that stick out where you can pick them out as a dub.
Good luck,
RD