As I said before I don't use callback. I'm new to maxscript and... Sync, your code is too complicated to me.
Here the steps that I use to get the last selected vertex:
1. selecet some vertices - 10 for example
2. start script
what script do:
2.1. store vertex selection in array
2.2. max undo
2.3 store vertex selection in a new array
2.4. find the last selected vertex - the one that is in the first array, but not in the second
2.5. max redo
Now I have the last selected vertex and can weld all selected vertices to it. I know this is not verry elegant and may be my method is wrog, but in Editable mesh object with 690 000+faces(346000+vertices) it takes less than 3 seconds to weld 10 vertices.
Bryan, you can use paint selection, but the last vertex must be selected with single mouse click.
Sorry for my english