OK, I looked up as much information and read as many anecdotes about the speakers being considered as I could find, and did some testing with a phantom center to refresh my "audio memory" as to its practical effects. Hopefully this will answer at least as many questions as it raises.
First let's go over the phantom center option and its ramifications. I watched (and rewatched and re-rewatched) a few center dialogue-heavy scenes from several movies, some with just dialogue and others with action going on simultaneously--all the while shuffling slowly from side-to-side like a crab.
To cut to the chase, it wasn't at all difficult in most cases to hear the "comb-filtering" effect, even after moving a foot or less to the side of the central sweet spot. Interestingly, at least to my hearing perception the intelligibility of some people's voices were affected the most at different positions from other people's voices, which implies different frequency ranges. Sometimes they sounded a bit muffled, and in other cases a bit quiet and/or recessed in the midrange, any of which could potentially cause intelligibility issues when the overall volume is sufficiently low. As I moved outward, farther off-axis, things sometimes actually got better, then worse, then better, then worse--how textbook! Note that I was listening critically, but at the same time it was easy to detect--in some spots it gave the impression that my speakers were noticeably lower in quality than they actually are, or that the soundtracks were more poorly recorded/mastered than they are. Also note that things cleared up quite a bit as I got way off-axis (I tested out to about 45 degrees) because at that point I was hearing mostly just the left or right front speaker (makes sense).
For the sake of comparison, I watched the same scenes using my vertically-oriented center speaker (the ideal configuration), and it was of course no contest. Nothing noticeably changed until I got quite far off-axis, at which point the higher frequencies were starting to drop off (my speakers' tweeters are of the ubiquitous 1" dome type), but there was still no effect on dialogue intelligibility or how the speaker sounded otherwise. By 45 degrees, the sound naturally wasn't nearly as "airy" as it normally is, but dialogue was essentially unaffected. From memory, with a horizontally-oriented MTM center (the most common type, which is sort of like two speakers placed closely together), typically the negative effects on dialogue intelligibility are more gradual out to about 15 degrees or so (give or take according to the specific design), by which point they really start to have a significant effect--this often yields (from, say, 10 feet away) one really good seat in the middle and one pretty decent seat on either side, with every other seat being a bigger compromise. Keep in mind that this is all relative, and that all of these configurations can work--some are just superior for off-axis viewers than others, and while you could ignore the negative effects on sound quality or compensate for them by turning up the volume (and probably "riding" the volume control the whole time), they can in fact be more pronounced than the differences between many makes and models of speaker, which implies that if you paid a lot for higher quality speakers and don't always watch movies alone, then it behooves you to try to accommodate a center speaker of comparable quality and capability if at all possible.
Alright, now let's look at the speakers in question. From nearly everything I've read about the Energy V-Mini in action, they are decidedly small
speakers, unfortunately. Their specs look somewhat impressive for a small speaker, with a 4.5" midwoofer (or two in the center speaker) and extension down to 78 Hz (-3 dB), but the truth is that their midwoofer is no larger than the 3.5" ones in the Cambridge S20 and S50, and the vast majority of people who use the V-Mini have to cross them over higher or else they start to sound thin real quick. That's about all we have a right to expect from speakers this small, but I was sort of hoping they'd be a little more "butch"
given their series pedigree, normal price, and their alleged 4.5" midwoofer. This system probably can work for you, but it's definitely small, with all of the tradeoffs and limitations that go along with this.
As for the Cambridge Audio speakers, the real standout is the well known (by now) and respected S30--it's larger than the V-Mini but still fairly small, and for its size this speaker is remarkably "butch" in its bass extension and "punchiness." Normally I'd recommend using a third S30 as a center--I practically sermonized on this here
, banging my fists on the pulpit and everything
--but in your case, you may be forced to settle on the S50, in which case you may have to cross over your center to the sub at a frequency higher than 80 Hz (probably 90-100 Hz), which would be a compromise in at least a couple of ways. I don't know firsthand which of these two systems would have "better" sound quality, the Cambridge or Energy systems, but personally I'd go for the Cambridge system because at least you'd have S30s as your left & right fronts, and they'll play like bigger speakers (to a point). Unfortunately, there are some additional issues. For a smallish speaker the S30 is not well suited for mounting--you'd either have to drill into them, for which there is little space on the back panel, or install a shelf or a visible mount like the VideoSecu MS56B
for each of them. It's doable, but it's more of a hassle and a compromise to aesthetics. You could also use the S20 instead, which can mount directly up against the wall on a single screw, but then you'd lose the advantages of the S30. Another potential issue is that the S50 is 8.7" deep and needs an additional inch or two, at the very least, of depth to give its rear ports some clearance (the V-Mini-C has the same issue, but its cabinet is only 5.5" deep), so I hope that your mantel can accommodate this.
Obviously I'll need more feedback from you at this point (also ask any questions that you may have about what I've said here). If you end up deciding to go with a phantom center, for one reason or another, then obviously more options will be available for your left & right front speakers, but there is a price to pay for this, as described above.