Ask yourself the question, Where does any attractive force come from? What causes any attractive force to work?
Repulsive forces are easy. You throw particles at an object and push it away. You have a toy boat stuck in the middle of a pool. You throw things at it that land just short of the boat (try not to hit it and break it) and the waves you create push it the other side of the pool. But now stand on one side of the pool and throw things at it to create an attractive force pulling it to you. You can’t! Okay, if you threw a string over the boat with a grappling hook, you could pull it to you. Strings create the attractive force.
What creates the attractive force in strings? It is the vacuum, exactly like you described it. Two pieces of matter are separated and they want to come back together because of the vacuum pressing them together from the outside. The vacuum is nothing. Nothing doesn’t contain the concept of space. If you separate two pieces of matter but there is no space to separate them into, they will be pressed back together.
It is harder to answer, "Where does ANY attractive force come from?" Than to answer, "What is gravity?"
Once we realize how the attractive force, like the strong force, and electromagnetic ATTRACTION are created... Unfortunately, we take attractive force for granted. A proton attracts an electron. Why? What is attraction?
It is due to matter being pressed into nothingness. Nothingness is a concept that doesn't even contain the idea of space. Matter contains the idea that it is. If you try to move it, it resists, because it is. If you cut matter in half, and separate the two halves you are separating what is, which are the two pieces of matter when they are together, into what is not, which is the space necessary to accommodate two pieces of matter separated by a distance. When you try to separate the two pieces of matter into no space, the idea of no space resists being encroached upon, just like the idea of inertia (what is) resists being moved.
But I can't clearly explain the type of matter I am talking about. It is the dark matter that makes up all space. Light matter is dark matter that is being energized. There is a huge attraction between the points of dark matter. There is a strong attractive force thoughout the whole universe. It makes space like a quivering Slinky that can carry waves. Gravity is also an attractive force throughout the whole universe. It would have to come from that other attractive force, since they both act in the same direction. We don't think of the strong force in molecules as being the result of a strong attractive force throughout space, but there is no inherent attractive force in particles, there is only inertia. Inertia comes from the concept of what is, which is the only idea inherent in matter. The strong force in strings comes from separating dark matter into what is not.