The old insulation should have had a vapor barrier between itself and the floor of the attic. However it's possible that if it was blown in that it didn't have a vapor barrier - or someone goofed and put in unfaced fiberglass. I agree it probably would be a very good idea, if the old insulation is not faced, to pull it all up and add some vapor barrier underneath it all. A home can add a huge amount of water vapor to an attic space if there is no vapor barrier.
When I insulated the attic in this '41 home, the attic has zero insulation. I was using Miraflex batts but they had both sides perforated, so I somewhat laboriously cut polyethylene plastic into strips and laid them into the bays between the ceiling joists, and then slid the bats on top of them. This had a side benefit of easing intallation, because the ceiling was of an old design with hard plaster sticking up through holes in the ceiling boards, which otherwise would have snagged the batts when I tried to slide them into place.
I was mindful of the fact that adding insulation to the attic floor would make that space colder in the winter, bringing on the possibility of humidity condensing in the attic and dripping. So that was another reason for making sure there was a vapor barrier under all the insulation. Additionally, the soffits were all enclosed. Again, rather laboriously, I opened up a 2" wide channel in all the enclosed soffits, protected it with slotted soffit ventilation strips, and also drilled three 1" holes in the blocking in each rafter bay between the opened soffits and the attic. I also ran ventilation channels in every rafter bay, to prevent the fiberglass insulation (which eventually rose to about 16" at its tallest) from blocking the ventilation from the opened soffits. The result has been no sign of condensation in the attic. And of course a much warmer and more comfortable living space below.
Later I covered up the entire crawl space with more poly plastic, to reduce the amount of moisture entering the structure from the earth. There was also an old furnace chimney that extended from the crawl space and was left open in the attic. While this probably helped ventilate the crawl space, there was already plenty of vents in the foundation, and all it probably did was introduce more moisture into the attic. So I sealed that off top and bottom.
