The flight director will command exactly what you tell it to, so when you select V/S +300 the pitch bar will command a climb at 300fpm. With takeoff thrust on one engine climbing at 300fpm you will accelerate away from the commanded "speed window" speed and therefore you can commence flap retraction. After flap retraction you select FLCH and this resets the MCP speed to whatever the current speed is (just be careful that it is the speed you want, it may be a few knots slow or fast depending on when you pressed the button).
Hope this helps.