About the only cheaper phone service than that VMUSA $5 plan is
Freedompop, which can be absolutely free at that level of usage, if the account is set up correctly. It only works on smartphones and that requires you to be a bit technically savvy, though.
My parents have 2 phones with that VM plan that you have, OP. They like simple phones and will readily admit that they're not not savvy enough to handle Freedompop or smartphones.
If the accumulating balance is worrying you, my suggestion is to spend some of your account balance on a new phone.
VM will let you do that.