Its a hardware synth integrated into the VS-700 which is connected via USB. Therefore, it should be available
Did you install the VS-700 driver from the CD or download the latest version. Make sure your USB cable is unplugged when doing this. Follow the instructions in the VS-700 Getting Started Guide carefully.
To use the Fantom VS, go to Insert-Softsynth and it should be available. If not, then you may have installed to a non-standard folder. If so, you'll have to tell Sonar where you put it and scan your VSTs again. Its always best to install Sonar to the default locations otherwise you could have problems.
If you're able to insert it to the synth rack, then go to a MIDI track and set the Fantom VS as the Output.
That should do it. Its all in the VS-700 manuals.