I would like to record a vocal into VoiceOne directly. Can I use the XLR jacks on the back for my microphone?
No, you will need a mic pre amp to use the VoiceOne effectively.
Almost any mixer that you can buy these days includes at least one mic pre amp. You can also buy standalone mic pre amps from various manufacturers, some that even output S/PDIF which would be ideal for connecting to the VoiceOne.