Just be mindful of the fact that if you close mic the vocal with the SM7 and you mic the banjo from a distance as
Mike suggests
(and that would be nice for the banjo) then the vocals are going to be in the banjo mics as well as the vocal mic. The integrity of the vocal sound may be compromised because the vocals that are in the banjo mics may not add well to the close mic vocal sound.
(It may be OK too) For better separation of the two the banjo mics may have to come closer to the banjo and point down
(towards the bottom part of the drum head under the players hand) away from the vocals a bit too. I am not sure you need two mics either to record the banjo. There may not be a lot of stereo going on there although a X/Y setup may give you some slight stereo image.
(further away it is yes but as you get closer perhaps less.)
Test recordings are going to help you find out the best positions for the banjo mics. They do help to eliminate the uncertainty.
I quite like the way they have done things on those videos to pick up vocals and banjo at the same time. Like I said before I would try an X/Y near coincident setup turned sideways with one of the mics pointing upward toward the vocals and the other pointing down to the banjo. Again test recordings of this would determine if it is suitable. Otherwise go back to the vocal mic banjo mic idea.
I have recorded banjo quite close pointing down towards the area below the players hand and got excellent results. But I must admit there was a lot of other stuff going on and the clacking of the picks etc was not audible. It might be different in just a vocal/banjo recording.
I am sure between the excellent suggestions
Mike has made and some of my ideas you will get a decent result.