Welcome zest. Either the VP100 or VP150 will sound fine with the M60s. Please understand that no horizontal center channel will sound exactly like a floorstanding speaker. The different sized enclosures ensure that. The only way to guarantee a perfect match is to use the exact same speaker in all three positions. However, rest assured that pans across the M60s and VP150, or VP100, will be flawless.
And, don't be dismayed by how different the speakers sound, including the QS8s, when you run test tones (pink noise) through them when calibrating your system. That difference will not correlate to a significant difference when normal source material (movies, music) is being reproduced by the speakers. Many things, including that size difference, cause that difference in the way the pink noise sounds. In fact, due to the difference in surfaces near my two main speakers, when running test tones, my left main speaker sounds different from my right front speaker, and they are identical. With normal source material, I notice no difference whatsoever.
The primary difference between the two center channel speakers is that the VP150 is designed for larger rooms. It will play louder and has a slightly wider dispersion area. In a small to medium room, the VP100 will do fine.
If you'll give us an idea of the size of your room (LxWxH), and the configuration of your seating arrangement (i.e. approximately how far from the center channel people will be sitting, and how far off to the sides people will be sitting) we may be able to give you a suggestion as to which center might work best for you.
