The problem with definitions is that
(a) they can change over time as the subtleties of the design standards change
(b) there is often more of importance in the bits which the definition leaves out formally but which are built into the practical determination of whatever is under discussion during the certification exercise... it is this latter concern which makes fairly strict adherence to the manufacturer's AFM/FCOM procedures very important ... often there are comparatively critical things not stated specifically and which, as a result, the pilot cannot reasonably know about ...
The DC9 has a comparatively low Vmcg and, typically, DC9 pilots describe that Type as running on rails in the event of an engine failure ... don't believe it without some caution ... in the case of a very low V1 failure, the Diesel will skate like a beach ball on the waves in a strong wind ... I have some interesting runway flight test videos which demonstrate that characteristic quite convincingly.
If you lose an engine below Vmcg ? .. you close the throttles !! ... or else you run a very enhanced risk of departing the side of the runway and ending up in amongst the daisies ... again, the question is not answered simply as there are a number of things to take into consideration .. but the principle is set in concrete .. you stop !!
Mutt is better placed to speak to the 747 .. in general, with a derate takeoff (as opposed to using flex, or reduced, thrust), the V1 is able to be reduced as Vmcg is lower for the lower thrust output. Be mindful, though, that the selection of V1 for the typical FAR25 machine is not a simple matter and involves a number of considerations.