I would use your USB audio interface to record into a multitrack DAW program. Whatever you are the most familiar with is probably the best.

But record to .wav. Converting to MP3 should be your LAST step (keep those .wav's, because later on, if you get anything that plays audio, an arranger or a laptop/iPad etc., if you have enough room on it, the original .wav's will sound better).

Once you have recorded the sequence, then overdub any vocals you want (maybe get a guitar playing friend to come in and do some tasty rhythm!) and mix to the final .wav.

As a doublecheck, record yourself singing the lead, mix it in about right, and make sure your background vocals are mixed right (and any sequence part isn't either overpowering or getting lost in the mix) before you commit to that final mix.

If all your sequences are already tweaked so you don't have to mess about with the main volume of the sequence player on the gig, you can probably go ahead and record ALL the sequences without moving anything, which will give you a consistent volume on the gig.

Only after they ALL are recorded, and you feel they are consistent volumes, and ready to go would I convert them to MP3. There are many programs that will do this in one batch for you (even iTunes can do this), so you can set it going and go and have dinner and come back when it is done.

Last advice I would give is, if what you are playing the MP3's on has a decent sized memory (or reads them direct from a stick), go as high on the MP3 encoding as you can... 256-320kbps MP3 files sound SO MUCH better than squeezing them into 128 or 160kbps files! They take up twice as much room, but room isn't the problem it used to be...

Hope this helps.