Might be some confusion over the word Thrust in
Power = Thrust x Time
and the word Thrust in the Vx equation
If an engine produces 250HP of power for 5 hours with no propeller, how much thrust did it have? none? how can the equation be true then?
In the first equation, Power = Thrust x Time is not thrust of the airplane through the air, but thrust of the engine.
In the second equation, Thrust is thrust through the air (propeller thrust), not thrust of the engine.
Thrust through the air, or propeller thrust, is related to the efficiency of the propeller, as well as the power of the engine.