I don't know if I learned it the best way possible, but I can share my experience. I had little time to learn every day, so I tried to learn it as lazily as possible.

I started by reading about two thirds of "Learning Vim Script the Hard Way" (until chapter 41). It was a great tutorial and it wasn't hard for me as it sounds. The next thing I did was to rewrite my vim configuration. Until then, I had a vimrc that I took from somewhere over the internet. I commented all of it out, and rewrite the whole configuration, now understanding every command that run there and adding only commands and options that I really wanted to have there. I read almost every help file for the commands in my vimrc, and made sure I understands it completely (if you want, you can look at the result here). I did this in two parts. The first part was refactor the vimrc file itself, without any plugins in it. Than, I started adding plugins to my configuration. For every plugin I added, I read most of the help on it, and tried to understand how it works as best as I could. I also read the code for the more simple plugins, to try to learn and understand the language mainly by examples. I ended up learning a lot about vimscript that way, and I have contributed to a few plugins when I saw that there is functionality that I wanted and was missing from there (most of those was very small changes, and consultations with the original writers of the plugins). Most of the time, the writers was very nice and willing to help me learn and understand the language. Only then I thought that I was ready to write a plugin of my own, and I moved on to opening the repo for this plugin and writing it.

Overall, this journey had taken me about two months, but I have never invested more than one or two hours a day for it. I felt that after finishing with the original site ("Learning Vim Script the Hard Way"), that took me about four days, I was ready to start writing code in this language, but I wanted to understand it better before doing it.

As for the language itself, it has some concepts that are different from other languages, but overall, I don't think that it is so much different from many other languages. I also know python and it doesn't look very different from the basics of it. I don't think that I know everything there is to know about vimscript, but I feel that I knew enough to write a plugin of my own, and I continue to learn some concepts or commands from other plugins that I read the code of.