Couple of thoughts ..
(a) the Vmc concern is only relevant for rather low weights as, for higher weights, Vs becomes limiting.
(b) on whatever runway RTOW, Ops Manual, QRH, etc., data you have you should be able to locate a chart of some sort which gives either specific or generic speed schedule data against weight.
From the higher weights you should see the speed schedules reducing with weight. If, at the lower weight end, you see the speed schedules flatten out, that will be where Vmc becomes limiting and you should be able to divine an approximate Vmc value, even if it is not stated explicitly.
If the weight/speed schedule continues to reduce down to the minimum weights, it follows that Vmc is below the scheduled speeds. How far ? Good question if the figures aren't listed somewhere explicitly. In this case, I would refer the matter to your ops eng folk .. who either will know the figure or can get it from the OEM.
I would opine that the more important concern, if you do elect to increase thrust at low speed on a low weight takeoff .. do it SLOWLY. I was involved in the investigation of a fatal many years ago where, almost certainly, the departure was caused by the pilot's rapidly increasing thrust on the operating engine .. both engines were high spec'd and there would have been a thrust overshoot .. which led to a very rapid Vmc departure over the runway head .. crash, burn, most died.