I am experiencing some odd behaviour in my rig where vertices with
0 weight are moving when they shouldn't be. I have tried painting different weight onto those vertices but it does not seems to help.
There could be two solutions to this problem. However, you can fix this problem easily. Solution #1 Go into the vertex groups menu, select the vertex group For the bone that is causing your problem, go into edit mode for your character's object, select only the vertices you don't want to be effected by the bone, and press remove. this will remove any un-wanted vertices from the bone's influence.
Pressing assign will add any selected vertices to the vertex group with 100% influence. Pressing remove will remove any selected vertices from the vertex group's influence
Solution #2 You have no weight for the vertex causing the problem on any of your foot bones. Thus, when you move the Thigh L bone, it moves the lower leg and all the foot bones with it, but because that single vertex has no weight, no bones will move it. to fix this, go into weight paint mode, select an appropriate foot bone, and paint on some weight. Or you can select that vertex, then select the bone's vertex group and click assign.