Vtoss couldn't be simpler to explain; it is the lowest speed at which, with one engine inoperative, the aircraft is able to climb. When providing the first segment climb to 200ft, the manufacturer will stipulate the speed required (it may be the speed used for Vtoss which, unlike Vy, will change with the mass of the helicopter).
The best angle of climb is achieved with zero speed and for that reason, it is rarely if ever provided. As Shawn has said, any speed used in the CAT A procedures must be achievable using the airspeed indicator.
We have been waiting for the definitive explanation to the question posed at the start of this thread; I thought HeliTester was going to provide it but he/she didn't so I might hazard a guess:
When the helicopter reaches TDP, unless it is being flown in an aeroplane type runway manoeuvre (i.e. a level acceleration until V1 is reached), it will require an attitude change. If the engine fails at (or just before) TDP the inertia that is stored in the rotor - due to the beeped-up speed - is used to achieve the attitude change without too much height loss. As soon at the resulting (stable) attitude has been achieved the drooped rotor is permitted to recover to the most efficient setting, and the climb commenced.
For helidecks and for elevated heliports, the rotation-and-droop should result in deck-edge miss.
Jim