Using Zinc as the reducing agent

Let's look at the first stage of the reduction - from VO₂⁺ to VO²⁺. The redox potential for the vanadium half-reaction is given by:

The corresponding equilibrium for the zinc is:

The simple principle is that if you couple two of these half-reactions together, the one with the more positive E° value will move to the right; the one with the more negative (or less positive) E° moves to the left. If you mix together zinc and VO₂⁺ ions in the presence of acid to provide the H⁺ ions:

That converts the two equilibria into two one-way reactions. You can write these down and combine them to give the ionic equation for the reaction if you want to.