I recommend that you use AutoHotkey for this.

You can do a per-application hotkey change:

SetTitleMatchMode,2
#IfWinActive,VIM
   CAPSLOCK::ESC
return

#IfWinActive
   CAPSLOCK::CTRL
return

This script, for example sets caps to escape in vim, and control everywhere else.

There's no vim key identifier for the capslock key, so you have to do it with a scan code.

It would be like trying to map the ctrl key or the shift key to something - those keys don't do anything by themselves, they are key modifiers, so they can't be trapped the same way.

The mapping has to be done at the OS level instead of vim, so the complication comes from the OS. For example, on my Ubuntu machine I have the following in ~/.Xmodmap:

! Esc on caps lock
remove Lock = Caps_Lock
keysym Caps_Lock = Escape

However, I don't think the REGEDIT script is so bad. It's kind of ugly, but it's only two lines!

Remap CAPSLOCK to ESC and CTRL on Windows

A very nice tool that I've already described in this answer gives you the possibility to remap CAPSLOCK to both

1. to ESC (when pressed alone) and
2. to CTRL (when pressed with other keys)

Just use this little piece of open source software by ililim.

You do not need Admin privileges for this and ESC and CTRL are still working as expected.

I use it to enjoy convenient CTRL + anything presses without hurting my pinky and to toggle modes in Vim that I use via ssh in MobaXterm. For installation just follow this description.