I think its cuz u have wifi preferred on network settings, so when u turn ur wifi on, the phone uses the wifi connection and blocks out the data plan...and vlingo only works by using the data plan. That's the only reason I can think of
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I had the same thing and I finally gave up. I did try to keep it on only data. Then I tried 3.0 and it worked on wifi preferred but I will have to pay to use it with non-BB users so I am trying to decide if it's worth it.