My simple question is, would Cubase allow me to switch drivers, so that it will automatically make saffire pro as my audio interface and record vocals without removing firewire cables and rebooting machines in the middle of a recording session?

Cubase will allow you to "hot swap" the device you wish to use as your audio interface without having to reboot your computer or the program mid-session. (If your computer is a Macintosh you can simply setup on aggregate audio device so both devices can be used without the need for swapping ASIO drivers). If you are on a Windows computer you must go to DEVICES > DEVICE SETUP > VST AUDIO SYSTEM > and select the device you wish to be your audio interface as your single ASIO driver.

You don't mention this but you must consider it as well: the device acting as your audio interface is responsible for getting audio into and back out of the computer. In order to monitor (listen to) what you are doing, your sound system must be connected to the device currently acting as your audio interface. So "automatically" is not a word I would have chosen for this swap. Add to this the complication of: if you are playing MIDI tracks or you are playing the XS directly, for example, while the vocalists is overdubbing, then in order for the vocalist to hear the XS, the XS must additionally be connected to inputs on the current audio interface. If you have already rendered the XS Tracks as audio, then no worries.

Secondly, the music track I recorded (from motif xs8) in cubase can be outputed to saffire pro audio interface (or any audio interface) so that vocalist will just use this as a playback when recording his/her vocals?

Of course, the device acting as audio interface does not care where the audio originated. It simply plays back audio. But as mentioned above it must be audio tracks playing back. If you have already rendered the XS data as audio, it will playback through the device you have setup in VST AUDIO SYSTEM as your ASIO driver. If, however, the XS is generating audio in real time (as in you playing, or it's playing back MIDI tracks, then you must connect it too, to your audio interface (Saffire) - so make sure you get a model capable of handling both the keyboards inputs and your microphone inputs.

One other very important thing, simply connecting your speakers to the audio interface does not always solve all issues in a typical home studio. In most home recording situations, the Control Room and the Studio Room are one and the same. Many folks new to this learn the hard way exactly why there is a soundproof wall between these two rooms with a large double glass window... If your microphone is "live" and within earshot of your speakers, you will have an acoustical feedback situation. The singer cannot be singing in the same room with the speakers.

So what you must do, in the ideal situation, is connect the audio outputs of the Saffire to whatever you are using to power headphones... Meaning you do not necessarily have to swap speaker leads. (A separate headphone system, or connect a couple of headphones some how to the audio interface (at least for yourself and the vocalist). The room with the microphone must be acoustically silent! Make sense?