I dunno about Fallout 3 Body meshes, But Know quite much in Skyrim body meshes. I can only use my experience in Skyrim and also with 3Dmax. However the connected vertex between body and neck is not enough, this is because the skinning use by some bones even a small affected which cause the seam, so I always use the maximum vertex weight in between that seams, linked just to a single bone, so i have to check all other bone which may joining the skin/rig such as clavile, spine1,etc.