How do I vary the playback speed in VLC using keyboard shortcuts?
VLC has a wonderful short cuts menu where you can see or even set the shortcuts.

Open the VLC preferences and the Hotkeys tab.

Restart VLC after making the hotkey change.

The following was tested by me on VLC Version 2.0.9 Twoflower (Intel 64bit).



Ranting:
I wish every app would have such easy access and feature, rather them me searching the Google to find out what Hotkeys they use and how to set them.
End of Rant.