"みくよん Miku Yon" is a fan made Miku manga written by Nagimiso. It's almost silent manga, so maybe easy to understand for non-Japanese speakers.
The first episode of "Miku Yon" was uploaded on Nov 30 2007, and it seemed like an ordinary short funny manga. But from episode 8, it started to show its profound story line and well defined background.
I don't think Travolta P (the creator of Kokoro) got idea from Miku Yon. The question "Can a doll (or robot) gain a mind?" has been tackled by many artists. For example in Ghost in the Shell, Director Oshii Mamoru showed his opinion on this issue by presenting the "Puppet Master", which is an android with "Ghost (kind of mind)" in it.
Anyway, if you like Miku Yon, then try other VOCALOID manga above. They haven't finished yet, but they are worth reading.