Is "ran ran ru" a reference to anything?

A Japanese McDonalds commercial, I believe.

Ran ran ru was a video of a Japanese workout video featuring Ronald McDonald set to the Touhou song UN Owen was her.

http://www.youtube.com/watch?v=Nhizo7KrZrw&feature=related

Not quite... Anonymous #1 is correct.

http://www.youtube.com/watch?v=OiwxMxzCPNg

and what Vocaloid is actually singing in the first panel is Meruto, quite famous single.

http://www.youtube.com/watch?v=m4jkxXyCBgs

Isn't "ruu" with raised hands a Lucy Maria Misora greeting? What's the connection (if any)?

There isn't any connection between her greeting and Ronald Mcdonald's promotional exercise dance routine.
Hence the parody picture post #57555
and if there ever IS a connection, I would like to see it.

nananonina said:
and what Vocaloid is actually singing in the first panel is Meruto, quite famous single.

No, it's Melt. It always has been and always will be, just because it's Japanese doesn't change that.