Sad to see this opened for 3+ years and still no official reply
Well here is a way to hackily do it, open your web console and paste in highlighted content from
https://github.com/codemirror/CodeMirror/blob/ab25dd7582f4954fc05892c3a85f592fad509925/keymap/vim.js#L47-L5499
after that, also execute
quanto.editor.doc.cm.options['keyMap'] = 'vim'
to retroactively set the keymap. And now you should have a editor that is in insert mode.
I also think you can automate this process via some browser plugin, but I just want to get started with the platform first :)
EDIT: If you have a blank document, it's not going to exit insert mode correctly, type something first!
EDIT 2: I have added some keybindnigs that I use for CodeMirror + a fix to handle the vim keymap initialisation better - you can find it here https://github.com/ryantam626/quantopian-vim-hack
EDIT 3: Looks like :
in visual mode behaves weirdly, that's a bit annoying..