Assuming that you are very much new to Vam. The models in Vam are for most of the time created in Vam itself, but you can port 3d models from other software into Vam. Most used outside of Vam will be Daz 3D That's the main and must have and then there's Blender, but you can use any other apps most likely. Blender is just free.
Vam for creating Vam characters or edit DAZ models.
DAZ for creating characters and morphs that are imported into Vam.
Blender, Maya etc. for taking other 3D models like game characters or your own created morphs to import into Daz to import into Vam.