What we've been trying to explain is that you don't need to do any of this. ;-) If you have V2P4, then you have a figure who can wear Posette clothing, but have the full set of Vicki 2 head morphs. If you would prefer Vicki 3's head, just download the V3P4 figure from Netherworks. The method you're describing will only ever give you "reasonable" results at best. There will be gaps, maybe visible seams where you join the two meshes together. You would need to involve a 3D modelling program to get it right. But why bother when others have done the work for you?