I think that if I had to pick one singular mic that was good in general for both
acoustic guitar and vocal I would get a used CAD E-100.
I actually prefer the neutral flavor it has on Acoustic Guitar and is my "go to" mic for it anyway. It does not have a hyped upper end so it works well on vocals even if they are a little thin in the upper ranges.
On the other hand, since it is a mid-sized diaphram it does not exhibit the low end proximity boost you may want from a large diaphram mic.
Another mic that is good but more vocal oriented than Guitar oriented (it may be a little bright or boomy depending on placement) is a MXL V67.
I have a Studio Projects B3 (multi patterned version of B1) and I like it a lot as well.
It has a very hot output and is very clean sounding and not as bright as the "C Series" mics. If your
acoustic guitar has a harder edged sound to it you may not like the sound you get from the B1, but if it is a mellower sounding guitar, the B1 may compliment it.
You know, as inexpensive as the Naiant mics are for recording acoustics, there is no reason to not have one mic tailored for each purpose.
my experiences naturally...