Install an SPD pedal, attach a shoe, and measure the position of the center of the shoe, right between the cleat holes. Install a Vector 3 pedal (on the other side), attach a shoe, measure, and compare. Measure from the outside surface of the crank arms. If there's a difference this will tell you what it is, and how much spacers to add. A 5mm difference should be obvious.

Phase differences and imbalances in cycling dynamics can also be due to leg length differences.