And yet most of the critical dialog and "action" comes from a center channel. I totally get the idea of a phantom center and it would probably be the smoothest transition of sound, but most people do admit to a lot of clarity when doing it because an entire channel of audio now has to be "mixed" with other audio channels and you can start to get "muddled" sound (probably only noticed if doing critical A/B listening tests though).
Then again, my statements aren't answering the question asked, so let me get back to that...
OK Josh. As you know, the VP180 is a huge beast. I love mine. Replaced a VP150 center and never looked back. (VP160 wasn't even mentioned at the time, and I like its design better, but the VP180 is quite impressive in what it can do.)
I have a projector with an acoustically transparent screen, so mine is at ear height.
With the VP150, I was using it with a non-AT screen, so it had to be above or below.
With V2 of my theater I mounted it just below the ceiling and above my screen, with V3 it was about a foot off of the floor and below that screen. So I've tried both, like rrlev suggested...
The sound seemed better when it was below the screen, and the VP150 turned "upside down" so that the angle of the top of the speaker pointed it upwards to the ears. I would think that, depending on your display height, that under the screen would be better. Typically, but more-so the larger the screen/TV gets, you want your eyes at about 1/3 up from the bottom of the screen when sitting in your viewing position. That is why under the screen/TV tends to work best.
Or unplug your VP150 and give the phantom center a spin. I know that you don't have active LFRs, so your mileage may vary.