In a gas pipeline, the high pressures help to stiffen the structure. In a hyperloop, you have very low pressure inside the pipe which changes the pipe design completely.
In fact, let's do the math. For a typical carbon steel tensile strength of 370MPa, internal pressure of 1200psi with a factor of safety of 2, and a radius of 2.5ft, you'd need 3.4cm thick walls:
2.5ft/(370MPa/(2 * 1200psi)) in cm
https://www.google.com/search?q=2.5ft/(370MPa/(2*1200psi))
According to this design guide for carbon steels, and using 30psi pressure differential for vacuum (i.e. a factor of safety of 2), we get: http://lss.fnal.gov/archive/test-tm/1000/fermilab-tm-1378.pd...
1.2cm thick walls, which is much thinner than the gas pipeline.
5ft * ((1-.28^2) * 30psi/(2 * 30 * 10^6psi))^(1/3) in cm
https://www.google.com/search?q=5ft*((1-.28^2)*30psi/(2*30*1...
So those gas pipelines should already be plenty thick enough for vacuum service. My point stands.