Where did it become known that Rin's and Len's names have different first letters? According to the answers here and this chat post


  there's one sound in Japanese which is somewhere between L and R, though probably closer to R.



Luffy/Ruffy (One Piece)
Lio/Rio (Kara no Kyoukai)


2 of the Vocaloids are the twins Rin and Len, but given the above, their names could be Rin and Ren, Lin and Len, or even Lin and Ren.

So where did it become known that Rin's and Len's names have different first letters, as opposed to the alternatives that I mentioned?
The earliest archived revision of Crypton's page for the Kagamines that reveals details about them has their names in both Latin script and in Japanese. 



That revision is from 3 Dec 2007, which is shortly before they were actually first sold later that same month. 

So I guess the answer to your question is that it was known to people pretty much from the very beginning.