Reply: Any time a person sits between two mics that are on, the 3:1 rule is violated, so some comb filtering is inevitable. Sorry, I have no experience with how the Vortex affects the 3:1 rule.
The spacing between microphones should be 2X the distance from microphone to mouth. This results in even coverage of all the people (approximately equal loudness). So in a room with an 8' ceiling, if the mics are mounted in the ceiling, they are about 4 feet from the participants' mouths. So the mics should be 8 feet apart.
The Signal-to-Noise Ratio of the PZM-10 is 68 dB at 94 dB SPL, 48 dB at 74 dB SPL (conversation at 1 foot), and 36 dB at 62 dB SPL (conversation at 4 feet). The noise referred to is the mic's electronic noise in a room with no acoustic background noise. If 36 dB S/N is acceptable, then the 4-foot miking distance (of mics on an 8-foot high ceiling) should work.
That figure is for a mic directly above a person speaking. People between two mics will be a little farther away, so the S/N will be a little poorer.
If 30 dB S/N is acceptable, then an 8-foot miking distance should work. We're assuming that the Vortex is eliminating most of the audible room reverberation.
If the ceiling is very high, you could hang some 2 ft x 2 ft clear plexiglass panels 4 feet over the table and mount PZMs in them.