One of the great things about Moonflower vines (Ipomoea alba) that I touched on in my first post about them is how prolific seed producers they are!
If you planted moonflowers this year, you should be seeing many seed pods. Most of the seedpods have probably not dried yet (at least most of the ones on my plant haven't) , but once they turn brown, just clip them off and place them in a Ziploc in a cool dark place. If you want vines in the same place next year, just leave a few seedpods and you'll have volunteers next year!
After the last frost in spring (May in VA), just plant the seeds about a half inch in fertile soil (add some potting soil to the clay). For a higher germination rate you can nick the seeds before planting, but over seeding works well. Keep the soil moist and soon you'll have moonflowers all over the place!
Here's a pic of my moonflower that I took about a month ago. It has spread about 10 feet down the side of the house since this picture was taken.