VMCG v V1 on Contaminated Runway
 

For contaminated ops, I get there is a V1 and Mass reduction but if the V1 falls below VMCG, then VMCG becomes V1. In this case, isn't stopping distance compromised and the risk of overrun reintroduced? Bit puzzled on this one. Cheers. KMB01

No. Your thinking IS correct, however there are more steps that need to follow.

If the V1 you wanted at first and needed to stay within ASDA falls below Vmcg and thus is unattainable, the take-off is not allowed. The risk you discovered is real and not acceptable.

To increase V1 back up to Vmcg you can reduce the take off mass (Ek = 1/2 v*v*m), and the Vmcg (unchanged) will be inside the valid V1 range (ASDA to TOD/RA)

And then there is, in some aircraft, the possibility to reduce thrust and get a lower Vmcg, which could enable take off again.

In other words you’re now weight limited when your V1 is increased to Vmcg

If V1 is Vmcg then you're at a light weight and will probably have a short ASDR and TODR assuming a balanced field...there should be little risk of an overrun....if my rusty memory is correct.


Edit: minor but important error to correct.
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Relating V1 with Vmcg in this discussion could be misleading - a decision speed with control characteristics required for continuing the takeoff, but not directly the distance required to stop.

Reduced thrust may not be authorised by your regulator. 'The AFM states, as a limitation, that take-offs utilising reduced take-off thrust settings – Are not authorised on runways contaminated with standing water, snow, slush, or ice, … ' (CS 25 ref)

Risk - uncertainty - CS 25 AMC 1591
8.1.1 Operation on runways contaminated with water, slush, snow, ice or other contaminants implies uncertainties with regard to runway friction and contaminant drag and therefore to the achievable performance and control of the aeroplane during take-off, since the actual conditions may not completely match the assumptions on which the performance information is based. Where possible, every effort should be made to ensure that the runway surface is cleared of any significant contamination.

8.1.3 The provision of performance information for contaminated runways should not be taken as implying that ground handling characteristics on these surfaces will be as good as can be achieved on dry or wet runways, in particular following engine failure, in crosswinds or when using reverse thrust.

"Risk is the amount of uncertainty which operators have to manage".

Do not ask "how can this be done", but "should I be doing it".
How - is theory; its the doing which involves risk.

That is related to assumed temperature or FLEX takeoffs, derated thrust is permitted.

Be mindful of canceling a Derate because of potential controllability issues at low speed... (There is technically no reason why you can't use rated thrust) This is because the OEI performance is predicated on the Derate when a Derate is used.