If I lower my Tyre pressures, will it mess with my VC function?

Si

 

Assuming you are running sanctioned wheels/tires no, not as long as you stay within the inflation pressure guidelines for the tires and the load you are carrying.

I would hazard a guess that if you managed to lower the tire pressures say at the front to the point the viscous coupling was at risk you'd find the car undriveable due to the excessively low tire pressures up front.

My experience with my 996 Turbo is it doesn't take much underinflation at either axle before the car's steering or handling or road feel have me heading in to check and adjust the pressures back to the recommended values.

 

i think as long as all tires stay within +/- 3% of each other you are fine.