...okay, I think it is best to consider Vmu as "Minimum Unstick speed", that is the lowest speed at which the aircraft will become airborne when you rotate. If you rotate at a speed lower than Vmu, you are in real danger of a tailstrike.
Vmu is 1) dependent on weight, typically an A320, 70tonnes=Vmu 140
60 tonnes=Vmu 130kt, etc, but check the actual data from your company.
2) Vmu is also dependent on pressure altitude, obviously if you are at a high altitude airport, the Vmu will be higher, as the less dense air has less lift.
You select TOGA if Vmu is limiting, because the speeds with Toga power, are higher, because you are getting the maximum performance available from the aircraft, and are not really looking at getting the benefit available from longer runways, which is reduced take off power, and maintenance savings.
Where I operate from, has 2133 metres of runway, and at weights of 64tonnes and more, Vmu is often limiting.
In summary, if speeds are limited by Vmu, first try a lower flex(say 4degrees lower), if Vmu is still limiting, then go Toga!
Hope this helps, spasibo!