Extracting Faces from a mesh causes new mesh to have unconnected vertices

[asalastrying]

so i have a body mesh i want to extract the face as a separate mesh, i go through this process:

1. select the faces of the mesh i want to separate

2. click Extract with keep separate extracted faces option checked with offset of 0

3. i check Keep Faces Together on, with the rest set to 0.

after that i get two meshes like i wanted the body and the face, but the face mesh has some random vertices that are unconnected now.

am i missing something in the process? if not how can i fix it? merging the unconnected vertices works but there are so many its not possible to go through them all to check 

[damaggio]

Just select them all and use the merge vertices tool on the face.

[asalastrying]

it doesnt really work, have to select the two unconnected vertices and merge them, which is fine but the problem is that i cant see which vertices arent connected till i check manually.

[artfully3d · Accepted solution]

Vertex mode, on the piece that has disconnected vertices.

And as @damaggio said, just select literally all of them --> Drag select over the entire piece

Then do a merge operation, if you set your merge value small enough, this will catch any overlapping vertices and reconnect them.

[asalastrying]

yes thank you both for the help!
the problem i was having is double vertices and i was setting the threshold for merging too large or too small.