Vanilla head to Citrus conversion

You don't import vanilla face sculpt into high poly head, period. Vanilla head and high poly head don't have the same vertex count so your scult can't be imported.
What you need to do is export your head shape sculpt into an obj file via Outfit Studio. Then import vanilla head and high poly head (both without any sculpting) into Outfit Studio, set the vanilla head as reference shape, make a slider and set it down all the way to 0. Once done, import obj file above as slider data -> Conform all -> pull the slider back up to 100 and hit export.
You need to use NifMerge to merge your newly exported head to your old head sculpt. Make sure to back up that sculpt before merging.
Tick Merge Child Nodes, set the type of both source head and destination head as "Face" and hit Merge to Vertex.
Once done, go into the Head tab of Racemenu, on Face Part slider, move it to the right until you get the high poly head, then import your merged sculpt