I don't doubt that water vapor can migrate into a Nylofume (nylon) bag. But let's look at the numbers.
A 24x36 Nylofume bag, tied off 4" from the top, and assumed to take the shape of a cylinder will have about 3.4 cubic feet of volume, or about 0.10 cubic meters of volume.
One cubic meter of air at 15°C (~60°F) weighs about 1.225 kg, so 0.10 cubic meters weighs about 0.1225 kg.
In a worst case scenario of 100% relative humidity, a kilogram of air holds about 10 grams of water. So, if we start with Zero relative humidity, a bag containing 0.1225 kg of air could, possibly, gain 1.225 grams of water vapor.
However, this number will be reduced –
If you aren't using the Entire bag - all 100 liters (6000 cubic inches).
If you reduce the volume of air in the bag by filling it with stuff.
If the gear already has trapped moisture, thereby reducing the humidity differential.
If the relative humidity of the outside air falls due to daytime heating, and the bag "expires" water vapor.
And last, at the end of the day when you open the bag, IF there is a gram (0.2 teaspoons) of water vapor trapped in your gear, I'm pretty sure your body heat will cook it out.
I don't believe water vapor migration is a significant issue.
A Nylofume bag will do quite well as a pack liner, as well as an odor proof bag.