Just because it says it is connected to BIS that doesn't mean that its not using wifi. When you are connected to wifi your device accesses BIS through wifi and not your mobile data plan.
So if Vlingo is connected via BIS and you have an active wifi connection with the wifi symbol being a solid white, then it is using wifi.
You can test this out by disabling your mobile data connection and seeing if it still works with only wifi enabled.
Posted from my CrackBerry at wapforums.crackberry.com
Our weekend place is in an area that does not get cell service. But we're able to use our 9700's on wifi. Viigo will not work when the connection is set to "auto"; it still tries to connect via BIS. So, when whenever we're up there, I have to change the settings to Wifi and all is well.