If you're a programmer, not hard at all. The language is fairly simple and quite powerful these days. It has a few quirks, like variable scoping (global, local, buffer, window, tab), but these are great once you get used to them.

The hard part is learning the API, which takes a lot of googling. Start off writing some helper functions for your own use and turn them into plugins later.

Not that much to be honest, there are some quirks and such as variable scopings (g:, s:, l: a:) and other things, but other than that it's pretty familiar.

A few great resources I found after having googled a ton around was [0] and [1]. If you are very familiar with vim (I'm not really a power user tbh), then all the commands you use with : are supported in vim script. Meaning opening a split in your script is, well, just split.

[0] http://ricostacruz.com/cheatsheets/vimscript.html

[1] http://www.ibm.com/developerworks/library/l-vim-script-1/

That cheatsheet is really well done. Thanks for that.

I learnt it by writing a befunge interpreter: https://github.com/serprex/Befunge/blob/master/bef.vim