AnFi,
As JimL rightly points out: the manufacturer gives a speed which is USED as Vtoss.
It obviously is not going to actually be: "the lowest speed at which, with one engine inoperative, the aircraft is able to climb [>100fpm]" for the conditions of weight and wind on the day - is it?
If you are loaded to the maximum Category A weight for altitude and temperature, and the WAT curve is based on the first takeoff segment (between TDP and 200 ft AGL), then yes, the manufacturer’s specified Vtoss is the lowest speed at which the aircraft is able to climb OEI at 100 fpm. However, the climb angle at Vtoss will improve with increasing headwinds, and the flight manual will provide a 1st takeoff segment OEI climb gradient chart (mean height gained in 100 ft horizontal distance) showing climb gradient as a function of weight, altitude, temperature, and headwind for the specified Vtoss.
Some helicopters have a selectable Vtoss, so if you have an obstacle in your path that you need to clear, you can select a lower Vtoss along with the reduced weight that goes along with that Vtoss. You can determine the required climb gradient from the takeoff site geometry, then enter the OEI climb gradient chart with the altitude, temperature, headwind, and required gradient to determine the combinations of Vtoss and gross weight that will produce the required gradient.
HT