Instead of additional mass, you could add additional energy. For example, a small maglev running around the inside of the vacuum sphere would steady outward pr

Instead of additional mass, you could add additional energy. For example, a small maglev running around the inside of the vacuum sphere would steady outward pressure on that part of the sphere. Add more trains at different angles to even out the forces. As a bonus, this kind of "active structure" can dynamically vary its structural strength as atmospheric pressure changes with altitude or weather. See Orbital Rings for more info.