Varispeed requires a workaround, but I use varispeed with Sonar a lot for vocals and the workaround isn't all that difficult. There are two main approaches for 64-bit systems.
Semitone variations are easiest. Create a premix of your tracks, and use the Tranpose DSP to transpose the premix. For example:
Transpose the premix down one semitone and sing along with it.
When done, transpose the vocal up a semitone for that "pop" sound, and delete the premix.
The transpose function sounds quite good unless you're transposing by huge amounts. I also use with guitar - pitch the premix up a semitone, play along, pitch down for a big, heavy guitar sound.
For finer transposition, open the premix as a groove clip but don't stretch to tempo. You can then use the Clip properties to adjust the tuning in cents. Same procedure as above - adjust tuning to the out of tune instrument or whatever, record, convert the new recording into a groove clip and transpose up by an equal amount. With such small amount of transposition, the audio quality is quite good.
If dealing with Groove clips is too messy, you can insert a pitch shifting plug-in like zPane's elastique for pitching down the premix and pitching up the new track.