vertices with similar weights not moving at same speed As the above image shows, whenever I move the armature that controls the upper arm, the corresponding part of the t-shirt moves slower than it. This is regardless of whether or not they have the same weights.
The picture shows them at different weights, but I tested to see if the same issue arises when all weights are the same. The whole object is composed of one mesh the t-shirt and the arm are all in the same mesh. How would I prevent the arm from moving through the t-shirt?
(I know I can delete the vertices that are blocked by the t-shirt, but this same error persists even then for the remaining vertices that are slightly inside).

I am using blender v2.72b.
Upon further playing around with the mesh, I found out that I should do the following to get what I want:


In edit mode, select the island of the cloth mesh, in this case, the t-shirt.
Press P to separate the cloth, click by "Selection".
Go to object mode, parent the rest of the mesh to the armature, with automatic weights. 
Then parent the cloth just separated to the armature too.
Go to pose mode and move the body, the cloth will follow it.
Go back to object mode, select the cloth then the body, and then join the two meshes, to get back the original mesh.
Going back to pose mode and moving the armature around still shows that the cloth follows the body's motion.


That's it, this worked for me and I hope someone will also find it helpful. If anyone has a different way of approaching this, please share.