Correct me if I'm wrong.
It seems to me that the relevant speed that applies on the approach phase (with regards to lateral controlability) is defined as "VMCL".
On my current type (ATR) "Vref" calculation is based on the greater between "1.3Vs" and "VMCL". Within the range of very light weights, VMCL is greater than 1.3Vs.
My understanding is that the discrimination between VMCL and VMCA lies on the flaps configuration:
VMCL: Landing flaps configuration.
VMCA: T/O flaps configuration.
For both speeds determination, L/G is retracted.
Then I guess that VMCL secures the lateral controlability, should one engine fail during the particular phase of G/A thrust application whilst the flaps are still travelling from LDG position to G/A position.
The slight nuance here being the fact that VMCA applies for T/O, when flaps are already set at the required fixed position, whereas the flaps retraction from Landing setting to Go-around setting during G/A phase had to be particularly covered, hence the VMCL notion.
That's why I find more appropriate to consider VMCL as opposed to VMCA during approach.
As for VMCG, I am not sure I understand its relevance during the approach phase.