In a vacuum triode, what prevents the grid from acting as another anode?
The grid is maintained at a negative voltage with respect to the cathode (similar to the operation of a N-channel JFET, or a depletion mode N-channel MOSFET), so electrons will be repelled by it. A result is that fewer electrons reach the anode, which has a positive voltage with respect to the cathode in order to attract electrons.
Whatever small current flows in the grid is considered leakage.