If we talk about the same thing:
It is the lower back PLUS the upper glute.
There are only some few custom morphs for that region.
"rotate hip", "pelvis front-back", "upper glute depth", .. I can't remember all that names... there might even one in my own "Small Body Morph Pack".
It is a bit difficult to find the right morph combination, and it is even more difficult to make it look good in every hip position. You can also fix this a little bit by turning the pelvis joint, but it is less than ideal.
In VaM 2.0 it will be a lot better, because it uses a system that automatically morphes/correcting a joint-region in an anatomical way while moving it.
You could also try the Joint Correct plugin and see, if it makes any difference for you. I don't know if it alters that body region, too.
JointCorrect is my amateurish corrective morph system for G2F figures in Virtamate 1.20 and later. I have no prior experience with JCM systems and only a fleeting understanding of anatomy. In my experience it usually produces better results...
hub.virtamate.com