I'm not familiar with Vision Cage products. I've heard some good and I've heard some bad things about their cages. I've heard that they warp and don't hold humidity very well. I have Reptile Basics 2x3 cages and am very pleased with them! I would buy another without hesitation if I were in the market for another cage.
With all that being said those dimensions would be fine for that size of ball python.
I have also heard good and bad on Visions, but you're going to find good and bad reviews on everything. Personally, my favorite acrylic cages are Animal Plastics. Have been working great for my animals. But anyways, Visions are perfectly fine.
To answer your question, those dimensions are perfect. Actually, that's just a few inches smaller than what I keep my BPs in.
Standard heat mats aren't going to heat through an acrylic cage. Flexwatt is definitely the way to go.
I've heard that they warp and don't hold humidity very well.
They hold humidity just fine, I have 14 of them and 9 of them hold ETBs, GTPs and Carpets. Emeralds and Green Trees need 80% humidity during the day and about 60% at night. If I'm not mistaken BPs require about 60% during the day. And as far as warping? I've not had one warp in the past 7 or so years. Who ever you "heard" your info from, they need to do a bit more research.
2.0 Pastel Ball
0.1 Spider Ball
3.2.1 Northern Emerald Tree Boa
0.1 Amazon Basin Emerald Tree Boa
2.0 Green Tree Python
2.1 IJ Carpet Python, one male a Jag