Shortcut for "vertical space" in LyX

Short answer:

The following command sequence inserts a \medskip in your document, you just have to bind it to the shortcut you prefer: inset-insert vspace medskip.

General answer:

At times it is a bit tricky to figure out what LyX commands one actually has to bind to a shortcut to achieve the desired behavior. While for most commands LyX shows the last executed command in the status bar, this does not help for commands that open a dialog for further settings. In the following, I describe the approach I used to figure this one out, so you could apply it for other questions in this respect:

(1) Start Lyx with the -dbg action option (that is, run /path/to/lyx-binary/lyx -dbg action from a console window).

(2) Execute the command for which you want to figure out the command sequence. LyX lists the actions in the debug output in the console. The following shows the relevant output for Insert->Formatting->Vertical Space... with selecting MedSkip:

LyXFunc::dispatch: cmd:  action: 219 [dialog-show-new-inset]  arg: 'vspace' x: 0 y: 0
LyXFunc.cpp(771): 
LyXFunc::dispatch: cmd:  action: 218 [dialog-show]  arg: 'vspace vspace defskip' x: 0 y: 0
LyXFunc.cpp(1842): dispatch msg is 
LyXFunc.cpp(1875): verbose dispatch msg (dialog-show-new-inset vspace)
LyXFunc.cpp(771): 
LyXFunc::dispatch: cmd:  action: 224 [inset-apply]  arg: 'vspace medskip' x: 0 y: 0
LyXFunc.cpp(771): 
LyXFunc::dispatch: cmd:  action: 225 [inset-insert]  arg: 'vspace medskip' x: 0 y: 0

(3) Usually the last command executed by some dialog-based action is the one you are looking for. In this case it is the line with [inset-insert] and the arguments vspace medskip.

(4) Use the LyX Mini-Command Buffer (View -> Toolbars -> Command Buffer) to manually test the command: inset-insert vspace medskip

If the result is what you want to achieve, you are done and have found the command sequence to bind to the shortcut. Otherwise you have to experiment more :-)

Update: General answer using the GUI only

For those who do not feel comfortable with starting LyX from a console window, there is also a GUI-only way to figure out the commands (thanks to Dominik for the hint!):

(1a) In LyX, open the debug Console by selecting View –> Message Pane

(1b) Configure the settings on the Settings page, so that User commands and External control interface are set to Yes:

(2) Switch on the Output pane and execute the command for which you want to figure out the command sequence. The following shows the relevant output for Insert->Formatting->Vertical Space... with selecting DefSkip:

(3) Proceed as above.