You could use a 200km evacuated tube. There is of course the issue of maintaining the vacuum as the vehicle exits. This could be managed using a pair of diaph

You could use a 200km evacuated tube. There is of course the issue of maintaining the vacuum as the vehicle exits. This could be managed using a pair of diaphragm shutters. Then only the space between the shutters would need to be reevacuated after a launch. Shutter failure would be disastrous.