If you are going to record keyboard and lyrics separately, that means that you'll be replaying the keyboard while recording the lyrics. An USB mic is only a recording interface, so you'll have to have a separate playback interface, and USB mics are primitive enough that you cannot synchronize them to external clocks. So your microphone recording will be recorded with a different clock than the replay of the keyboard recording happens and that means that your recordings will drift apart. This does not happen when recording with the same sound card you are recording on: in that case replay and recording clock are synchronous.
This is a recipe for headache, so I'd rather strongly suggest getting an external soundcard with microphone input (and headphone output, but that's pretty much available everywhere) and capable of full duplex recording at the quality you aim for, and then use a proper analog microphone rather than one having a built-in USB interface.
Many soundcards already come with some trial software for multitrack recording: that may be convenient to start with as well.