Your question, What did God mean when He created a vault between the waters to separate water from water?
Genesis 1:6 “And God said, Let there be a firmament in the midst of the waters, and let it divide the waters from the waters.”
I want to give you a similitude that might give you a picture of what this may have been like.
Imagine going to the center of the ocean separating the water to created a great circle. Everything with in the circle you would then call it the firmament.
Genesis 1:7 “And God made the firmament, and divided the waters which were under the firmament from the waters which were above the firmament: and it was so. And God called the firmament Heaven.”
But now when you created the firmament he left what was to become the earth covered with water. Where God separated the water on it forming the dry land which we see now. He called the dry land earth and the waters the seas.
The firmament, however, was devoid of any objects until God created the celestial bodies that we can observe with our eyes, including the sun, moon, stars, and planets. This did not take place until the fourth day.
Now it is written before God created what we now see in the firmament on the third day he created the following.
Genesis 1:11 “And God said, Let the earth bring forth grass, the herb yielding seed, and the fruit tree yielding fruit after his kind, whose seed is in itself, upon the earth: and it was so. And the earth brought forth grass, and herb yielding seed after his kind, and the tree yielding fruit, whose seed was in itself, after his kind: and God saw that it was good. And the evening and the morning were the third day.”
God did not create the things in the firmament until the fourth day. This means that there was no sun or moon in place for the grass, herbs, and trees to yield their fruit, whose growth depended neither on the sun nor the moon. (See comment for the rest)
