I'm using latest version of VS code (1.9.0).
After I undo (ctrl+z) something, and save the file (ctrl+s), then I cannot redo anymore (ctrl+y).
I would like to go back and forth during the editor is running same as Sublime or other editor. Can anyone solve this problem? Thank you.

Undo/redo on
a kept open file
That normally should work. If not, it could be an issue with one of your installed extensions. Try to start Visual Studio Code on a terminal/command line with
Credits: Issue comment from Benjamin Pasero
a reopened file
It's an unsolved bug (issue #7169). Vote & give feedback in this Github issue to help solving it. The VSCode developers track Github reactions to prioritize the work.
a saved as file
Also an unsolved bug (issue #17977).