John T-
According to an old Douglas letter I have, V1 cannot be less than Vmcg, Vr will be at least 5% above Vmca and V2 will at least be 10% above Vmca. These numbers are most likely the same as the regulations. In a worst case situation, lowwest seperation of speeds was 15 knots (This was just looking at the info I have home, I don't carry a full set of performace books around).
We use the American Airlines TPS programs slightly modified for our operation. Performance (and aispeed) margines using the TPS are increased by also using an Assumed Temp (AT, This is usually well above current temp) and Assumed Take Off Weight (ATOW, at least 2000lbs above planned wt). As long as you are below AT and ATOW you will have a wider margin between Vmca and V2 regardless of the power setting (reduced or max). You will still be able to meet the required take off and departure profiles but however at slightly less margins (runway used & altitude above) than if you use actual temp and weight & max power. For those numbers you need to get out the big performance book. (FO's hate it when I do this)
I hope I wrote that clearly enough. What kind of scenario are you using to set up the Vmc roll/yaw on take-off. Seems to me the pilot would have to rotate early and/ or abrutly and then increase the pitch to reduce the airspeed well below V2 all with the engine out. In other words really get behind the airplane.