'97 Encore with electric standby vacuum pump located in the avionics bay. The standby pump recently failed during preflight run up checks.   Disassembled the pump and found oil in the pump which had caused stuck vanes in the rotor.   The inside of the pump was oil wet. Where did the oil come from?      The primary engine driven is a dry vacuum pump.    Apparently there is enough oil in the line from the manifold where the two pumps connect  to the standby pump to cause problems. A