I have a Verizon BlackBerry for work, and a Sprint Evo for my personal. They are both with me at all times, so I can tell you about my Sprint roaming on Verizon.
If you have even one bar, or worse zero bars, but your Sprint device can still get a signal from a Sprint tower (no matter how faint) you will not be allowed to roam. So often I am near a Verizon tower and my BlackBerry has a full EVDO (3G) signal and my Sprint Evo will have zero bars and won't switch over. It's very frustrating.
Then here's the kicker...
After I finally get the phone to kick over to roaming after losing a Sprint signal altogether, I will then get a 1xRTT signal (2G) from some other roaming carrier...not Verizon. I will be sitting there with full bars 3G on my work Verizon phone and get 2 bars 1x on my personal Evo. This just happened to me this weekend where I was camping in Colorado. Very frustrated.
It seems that Sprint's PRL (Preferred Roaming List) is programmed that you roam on Verizon dead last. I love when I roam on Verizon (which happens very infrequently) because you get 3G data when you roam on their network, and the speeds are pretty quick. My guess is that Verizon's roaming is more costly to Sprint, so they intentionally make it the last carrier you will roam on.
Many people mistakenly think that their Sprint devices treat all roaming towers equally and connect to the one that is strongest. But unfortunately, it's not true. I would love to find a way to try to hack my phone's PRL and move Verizon up the list.
One thing I noticed that sometimes helps in trying to get off Sprint with zero bars and force it to roam is to go into airplane mode for about 5 seconds, then go back live. Most of the time it will go into roaming after doing this. Unless, you happen to move closer to the tower and get one bar. Airplane mode will not work if you have one bar of Sprint service.
Also note that if you exceed 350MB of roaming data. Sprint has the right to terminate your contract. It's in their Terms of Service. However, they keep this in their TOS to go after "permanent roamers" and not just to go after folks who occasionally go over their 350mb limit.