Ill try help you with the vert that is acting strange.

What you're going to have to do is use the component editor.

1)select the vert thats having the issue
2)open the component editor (window > general editors > component editor)
3)go to the "smooth skin tab"
4) here you will see the vert you have selected listed for the row
5) find the column that has the name of the joint you want it weighted to and set it to 1...this will cause all the other joints to go to 0 for that one vert


--here are some screen shots