The TC20 does seem like a good fit for yourselves but you might also consider the MC33.
Both of these devices are Android-only and due to lack of interest in Windows on our Mobile Computing line we have very few products that could run a VB app (maybe some tablets but nothing that would be a suitable upgrade path for you).
You are looking at rewriting your application I am afraid. A good overview of the available tools is I Need Tools For... - Zebra Technologies TechDocs - you might find using EMDK for Xamarin as the least steep learning curve. There are some online resources for "Visual Basic on Android" but I have never tried anything like that.