Just like how a jet engine uses the air it moves through to propel it self forward.
In this case energy for ionizing the propellant won't be needed because we already have ions.
How viable is my proposal?

Well, the concentration of particles in V-A belts is very low so your craft would need to spend a significant amount of time collecting ions. At the same time radiation levels in V-A belts are harmful to delicate electronics on the craft and so you'd want to minimise the exposure as much as possible. There's one contradiction.

 

Also, I'm not sure whether collection of ions in V-A belts is technically feasible or not.