The downward - vs - upward blade effects are commonly known as "P-factor", and is probably the dominant effect regarding Vmc.
I note the Lockheed P-38 used counterrotating props, but with the tips travelling OUTBOARD on top. This resulted in a very high Vmca, but allegedly improved combat manuverability.
Also note that if the vertical fin(s) is in the rotating propwash, a yawing moment will probably be observed. This is very common in SE prop aircraft and is what we usually call "torque". The fin will probably be offset to reduce this effect.