I keep the track as one, To me having more than one track for the same instrument or vox is messy and takes up screen space.
And first choice for me is to use Scook's utility and add a wave editor to the TOOLS menu.
You can use Sonars normalization but I find using Wave lab much faster. The reason being with Sonar ( and many wave editors) you have to keep re-opening the Process you wish to use over and over. With Wave Lab you open the Normalize or Gain process GUI , make a setting and it stays open until you close it. So easy to apply the same process multiple times.
I like Bassmans trick and will try that and see if it is faster. I just hate the way you have to use all those clicks to re do the same task in Sonar.
In Wave lab you can open that whole track even if it was a bunch of different clips, just highlight the entire track. If you zoom to the perfect size you will "SEE" the volume differences. I have often treated vocal tracks using the visual to look for peaks and low spots, While I'm in there I also silence breathing and any artifacts. If you get good at this you will have an almost perfect vocal track that may not even need much in the way of further processing to cut though a mix.
Some singers can't project low notes so they need a boost, working with applied gain works for me. I only use Volume envelopes when I'm feeling lazy :)