Yeah, I agree with what the others said - what I suggested will give you way wide vocals, and they'll be huge. But if you have existing vocals you want to widen them without actually wanting to record anything else, that's a different thing to what I was describing.
Automating chorus can be good, or a very carefully dialed in flanger with the feedback out of phase can do wonders. It's that phase cancellation in relation to the source tracks that'll give it the spread.
What I really like to do, to add extra width without the stacking, is run Waves Doubler 2 on a buss. Recreating that with stock SONAR effects, I'd put in a Sonitus delay on a buss (or Aux track), set left to 50ms, right to 55ms, no feedback, 100% wet. And then I'd run a Sonitus modulator with either the flange idea like I mentioned earlier, slow chorus or Ensemble, and have the balance more towards the wet end rather than the dry.
Then run a send on each of your vocal tracks to this doubler buss - for me it's usually around -12db send level but it's all to taste.
Basically you get a spread from the slapback delay and that's widened further from the modulation, but you're controlling how much you want to use this with the send from the track.
Works great on vocals (even a lead vocal can sound good with a bit added), clean guitars, synth pads... Heaps of good applications. :)