what next I agree completely.
The factor between fuel-load and payload makes mothership launching much more efficient than a vertical launch even when using conventional rocket technology. For very low payloads this is actually a practical possibility.
However the one big advantage of a vertical launch is that you can design space vehicles of upwards of 3000 tons (Saturn V). Realistically though a mothership is not going to be able to carry a craft weighing more than a few tens of tons.
So either the mothership or the rocket engine needs a huge technology advance.