I meant V2 when I said V2 in my previous post . V2 has to be above Vr , and VR and VMU are related, so indirectly V2 is related to VMU.-----> V2 must be above VMU
FCOM 2.02.14 P3 : " Check V2 against VMU limitation. If V2 is lower than V2 limited by VMU ,flex take off is not possible . Set TOGA thrust and retain the speeds associated with maximum permissible take off weight or the speeds read in the chart of the actual weight if they are all lower . "
I also recommend you to have a look at the diagram summary on FCOM 2.02.14 P8 :
V2> Minimum V2 ( VMU) ? If no---->flex take off not possible.
I also recommend to have a look at the table "V2 limited by VMU " FCOM 2.02.25 Page 2
VMU is the minimum speed at which the aircraft can safely lif off the ground. There are situations where V2 is limited by VMU . In that case V2 would have to be increased to minimum V2.
Example : FCOM 02.02.25 P4 : weight 60 t , pressure altitude 0 ---> minimun v2 limited by VMU is 124.
Again at low weights and altitudes , wet runways ..., Corrections applied on speeds make them lower than they should be . V1 might be less than VMCg, in this case V1 would have to be increased to VMCg .
If V1 is lower than VMCg in case of an engine failure , you will not keep the runway centerline.