I have a situation where, in a live performance, the orchestra started about 10% slower than it should have, and the conductor got the performance up to speed about the time the vocalist started to sing.  That means that the tempo changed on a curve, and I need to correct it on a curve.  I have an old version of Audition that would allow you to time stretch with an adjustable envelope, but it is old tech and does not do it without artifacts.  I am impressed with Sonar's stretch capability (I'm using X1 and/or X2), but I don't recall if there is a way to do it on a curve.  Essentially, I need the amount of stretch (shrink, in this case), to diminish right up to where the vocalist begins, to make appear as though the intro was played at the appropriate speed throughout. 
 
Any ideas?

If I think I'm following you correctly, you need to first set up a tempo map at the tempo change/rate that you want and then fit the audio to that tempo change/rate.
 
You can set up the tempo changes using Project-->Insert Series of Tempos. Once that's done open Audiosnap on the relevant audio and use "Clip follows project" to adjust the audio.
 
Remember that the bounced render will sound much better than the online render.

Another option would be to use the iZotope stretch algorithm in steps, like shrink it a little bit every measure. PITA, but it should do what you want. If the orchestra didn't notice they were starting off 10% too slow they probably won't notice the stepped changes 

The tempo map process looked the most promising, as I could map the tempo from a faster to the original tempo in increments making a curve.  I would do that after mixing down to stereo though, so I would not have to synchronize this change across 48 tracks.  How I actually did it was a bit more brute force, however...  The intro from the previous night was at the right tempo so I opened both projects and cut the intro (48 tracks) from the first night and pasted it into the second nights performance, nudged it just right, and voila (or as they say in Texas vo eela), a undetectable edit.  I love Sonar. 
 
Also, of interest.  A female member of a vocal trio was supposed to sing "This is the song the wise men sing", but sang the lyrics for the second chorus "this is the song the carolers sing", catching herself, and singing "ca-ise men sing".  An obvious goof.  I took the correct word from one of the male singers, pasted it in place, melodyned it to the right pitch, modified the formant slightly to make it sound a bit more girly, and again, a perfect fix that is undetectable in the mix!  Go Sonar.