Ctrl+D broken in MonoDevelop 5.7
Some of the navigation key bindings that used to page up/down the text editor have reverted to the default IDE functions e.g. Find in a recent build of MonoDevelop. It's probably time to take a look at issue 10.
Bump, this is still an issue in the latest build (MonoDevelop 5.8).
Ctrl+D is not broken per-se but was at some point used as a key chord for the debugger:

The workaround for now to get Ctrl+D working as before is to unbind these shortcuts. Click on the actions, clear the shortcut in the Edit Binding field, and click Apply.
I will look into adding the scroll down action in VimAddin as a rebindable action under the key bindings here. Then it will show up as a key conflict if this ever happens.