We explore vacuum as bonding force for modular robotics. Vacuubes are a set of modules that propagate vacuum across their interfaces in order to generate adhesive forces to form and hold structures. We use analog circuits to simulate vacuum transients and understand critical design parameters and then validate these insights in experiments. A 49-module structure that employs vacuum bonding is demonstrated. We conclude that vacuum offers a relatively strong, simple, reliable, and power-efficient connection principle.