I'm outside of VM service, but the people I'm with have Sprint and have full signal. So, unless it shows on VM's coverage map, you're not going to get service.
The common misconception here is that you'll get service where ever someone who has Sprint has service, but the truth is Sprint phones can roam off Sprint's network, so they'll have coverage outside Sprint's network. If you're on VM, you're limited to the Sprint network only, no roaming.
If you go outside their network, you can still use Wifi to get data, but if you want to talk on the phone you're stuck using something like GrooVeIP over Wifi.