Asuuming that moon plant and moon vine are the same---I'm not an expert but my neighbor has one that gets zero attention and looks fantastic. It's the one pictured in the bug on moon plant. I was awed by the blooms on it and she gave me a 8" stem with a few leafs in a cup with no dirt. It had not been watered for a week .I repotted in a 18" pot with MG potting mix. Thats been about a month ago and the plant is now about 30" tall and as much around.

Now, all did not go without a hitch.

The plant looked as you have described yours about 2 weeks ago. It had already grown considerably and I thought I was well on my way.
The problem I found was I had not removed the black plug in the bottom of the pot and the plant was basically drowning after a good rain.
I removed the plug ang dug the plant out and let the roots dry right on top of the soil in the pot for a day. Repotted, watered sparingly and the results are nothing short of miraculous. The plant has been fertilized once and that was with some tomato tone.
I have it in full sun all day.
From what I read thats all they need sun and a little water. I have no idea about saving the seeds. Sorry.

So my thought is too much water not enough sun.

Good luck.

I have never grown a moonvine in a pot, always in the ground. I don't see however why it can't be grown in a pot. I have some stumps in my yard and I usually plant them around the stumps to cover them. One year a dear friend, now deceased, brought me some rabbit manure. When I planted my moonvine that year I put a shovel full of the manure in the planting hole. It was an awesome sight to see when the vine flowered. We came home at dusk one evening and there where so many flowers opened you could not see the leaves. I have never seen anything so beautiful in my life. I have not been able to get one to bloom like that since.

Sharon