The VSonic GR07 BE is not too far off. But to dial it into the sound of the M100, you'll need to EQ.
I have the GR07 BE and the M100, so I gave it a shot, with a little from Tyll's frequency response curves, and dialed in a parametric EQ using Rockbox on an iPod. Here's what I came up with.
Frequency / Gain / Q
430 Hz / -7.5 dB / 0.6
5000 Hz / -7.5 dB / 0.6
Both gains are negative dB, which means you are pulling energy out of these frequencies. Going back and forth between the equalized IEM and the unequalized over-ear, the treble is still a bit off, but it sounds pretty close to me. That characteristic bass punch of the M100 is definitely there in the equalized IEM.
I don't know if you'll find an IEM that will be sonically identical to an over-the-ear headphone without EQ, though.
I hope this isn't too far off-topic. I figure getting the sound of the M100 is still in the spirit of the thread, right?