How to get skeletalMesh's vertex boneIndex

skeMeshCpt is USkeletalMeshComponent*
TArray softVertices;
skeMeshCpt->GetCPUSkinnedVertices(finalVertices, lodIndexRead);
FSkeletalMeshModel* skeletalMeshModel = skeMeshCpt->SkeletalMesh->GetImportedModel();
FSkeletalMeshLODModel& skeletalMeshLodModel = skeletalMeshModel->LODModels[lodIndexRead];

my character’s skeletalMesh animation is perfectly running In UE4 Editor and it has 4100 vertices, but I Find that softVertices result is error, there are 500 vertices’s BoneIndex 0(“RootBone”) I import this FBX into Unity, I Find no any vetices’s BoneIndex is 0(RootBone) in 4100 vertices