I don't think there is a problem with using Vermiculite in the tub with the Dame. However, I do have one question. The 80% humidity that you are measuring is coming from what location? I ask this, because if you do not have a probe of some sort inside of the snakes coils, then you are not correctly measuring the humidity that the eggs are incubating at. I say this, because snakes are very good at regulating incubation needs on their own. Some species of pythons actually vibrate the eggs with their body to generate heat. I am not aware of Ball Pythons doing that, hence the need to regulate temps during maternal incubation. However, Ball pythons, and other species of snakes do regulate humidity pretty good. How do they do that? Well if the humidity is a bit too high, they will loosen their coils to allow some of the humidity to escape. If the humidity is too low, they can urinate and tighten their coils to raise the humidity. With that said, I would still want the humidity to be as high as possible in the tub. However, "snake moms" have been doing a pretty good job at keeping their species alive by successfully incubating eggs for many thousands of years. They know how to do it.

As others have said, I would not risk adding water to the vermiculite. Mom will add her own if she feels the eggs need it. Just keep your temps between 86 and 90, and you should be ok.

I hope that helps,