Hm, I can't say I know much about how those old records were tracked...
However, in the modern era, I think there are 2 main things to consider (assuming you're recording acoustic guitar/singer stuff?)
First, I think you hit the nail on the head with the energy of recording both at the same time, especially if this is how the performer practices and plays most of the time. It can really add something to the tracks.
However, you might find that the guitar bleeds into the vocal mic, and vice versa. But, you mentioned only one mic. Again, Idk how they did this in the day, but I'd assume they used two mics, even if they only had 4 tracks to work with. Recording simultaneously could have gone either way, depending on the performer like I noted. The processing you would use for an acoustic is very unlikely to suit the voice and the same goes the opposite way.
In your case, I really don't think you want a guitar and voice on the same track. I just can't see it being very usable. You'd have to record from some distance away to capture them both, and in doing so, not capture much of anything good, especially in an untreated space (but I don't know what you're working with in that regard).
I'd say, use two mics, or record them separately.