I can't figure this one out. The drivers are the same and these center channels should work. I really have to wonder if the cabinets are undersized.

The VP-100 is essentially a M22ti:

Millennia M22ti Specifications
Max Amp Power 200 Watts
Min Amp Power 10 Watts
Freq Resp +/-3dB (Hz) 60-22K
Freq Resp +3dB/-9dB (Hz) 50-22K
SPL in Room 1w/1m 93 dB
Anechoic SPL 1w/1m 89 dB
Impedance 8 Ohms
Woofer 5.25" Dual
Tweeter 1"
Dimens. (in.)(HxWxD) 19.7 x 7.3 x 8.5
Weight (lbs) each 16 lbs

VP100 Specifications
Max Amp Power 200 Watts
Min Amp Power 10 Watts
Freq Resp +/-3dB (Hz) 95-22K
Freq Resp +3dB/-9dB (Hz) 65-22K
SPL in Room 1w/1m 93 dB
Anechoic SPL 1w/1m 89 dB
Impedance 8 Ohms
Woofer Dual 5.25"
Tweeter 1"
Dimens. (in.)(HxWxD) 7.5x17x7.5
Weight (lbs) each 14.1 lbs

Notice that the VP-100 is slightly smaller than the M22ti with the same driver compliment and with different frequency responses.

One of the best center channels I have ever heard was the Paradigm reference center. It is an unusually large center channel speaker (fact is it is huge for a center channel). What it tells me is that for those who would use a single full range speaker (if you had a pair of M60s and used another M60 as a center) you would have a very seamless front soundstage that you'd be happy with.

I think when center channel speakers first came out they were usually smaller than their full-range left and right counterparts. The main idea was to have e speaker that would reproduce dialogue and for the most part, that frequency range is very limited. theoretically most center channels should be adequate for this task. A lot of center channels ended up being anemic and not "full range". To my ears, Paradigm is one company that has done it right.

VP150 Specifications
Max Amp Power 400 Watts
Min Amp Power 10 Watts
Freq Resp +/-3dB (Hz) 85-22K
Freq Resp +3dB/-9dB (Hz) 50-22K
SPL in Room 1w/1m 95 dB
Anechoic SPL 1w/1m 91 dB
Impedance 6 Ohms
Woofer Triple 5.25"
Tweeter Dual 1"
Dimens. (in.)(HxWxD) 7.5x26.5x7.5
Weight (lbs) each 21.8 lbs

Paradigm Studio CC-570 Center
4-driver, 3-way Center Channel, MagnaShield™ (all finishes)
3rd-order electro-acoustic at 2.4 kHz 3rd-order electro-acoustic at 350 Hz (bass drivers)
25-mm (1 in) S-PAL™ satin-anodized pure-aluminum dome, ferro-fluid damped/cooled. Die-cast heatsink chassis, IMS/SHOCK-MOUNT™
102-mm (4 in) MLP™ mica-polymer cone, 25-mm (1 in) voice coil, ferro-fluid damped/cooled. AVS™ die-cast heatsink chassis, IMS/SHOCK-MOUNT™
–
Two 178-mm (7 in) mineral-filled polypropylene cones, 38-mm (1-1/2 in) voice coils. AVS™ die-cast heatsink chassis, IMS/SHOCK-MOUNT™
37 Hz (DIN)*

On-Axis (0°) ±2 dB from 60 Hz - 22 kHz
Off-Axis (30°) ±2 dB from 60 Hz - 20 kHz
91 dB / 88 dB
15 – 200 watts
150 watts**

Compatible with 8 ohms
29 L / 1.0 cu ft
24.1 x 67.3 x 33 cm
9-1/2 x 26-1/2 x 13 in
20.5 kg / 45 lb each

The Paradigm center is constructed essentially as a horizontal full-range speaker that is "similar" in function to a M60 speaker.

I wonder if it is a matter of increasig the size of the VP-150 enclosure or creating a new VP-200 (seems to be the buzzword around here for a new center) that is essentially a M60 converted to a center channel configuration.