Been thinking some more about this mystery of how Vizio is implementing its 'Clear Action Rate' and think I may have it figured out:
E-Series:60Hz native refresh panel with scanning backlight based on 2 strobing zones plus single black frame insertion for effective refresh rate of 180Hz (each pixel stays 'ON' for 5.6ms)
M-Series: 120Hz native refresh panel with scanning backlight based on 4 strobing zones pus dual black frame insertion for effective refresh rate of 720Hz (each pixel stays 'ON' for 1.4ms)
P-Series: 120Hz native refresh panel with scanning backlight based on 8 strobing zones (no black frame insertion) for effective refresh rate of 960Hz (each pixel stays 'ON' for 1.0ms)
R-Series: 120Hz native refresh panel with scanning backlight based on 8 strobing zones plus seven black frames inserted for effective refresh rate of 1800Hz (each pixel stays 'ON' for 0.6ms)
If this is correct, it means that neither the P Series nor the R Series supports a 'True Cinema Mode' of 48fps (which should not be possible on a 120Hz panel.
And assuming the Clear Action Rate of 1800Hz on the R Series is correct, it has probably been implemented by inserting a black frame in between each of the 8 strobing zones. I guess Vizio may have decided that they could not go to a full 1920Hz (or example by inserting 8 black frames of strobing 16 zones) because they did not want to sacrifice the additional panel brightness. As it is (and assuming my guess is correct), a Clear Action Rate of 1800 should mean that the panel light output is reduced to 6.7.% (1/15th) of what it could be if they were not using a strobing backlight.
If my guesswork above is accurate, it should mean that all of the Vizio panels beyond the E-Series ought to do a good job significantly reducing motion blur to near-plasma levels (and in fact, the Reference Series may have a level of motion blur below that of even the best plasmas...).