Well, if I had developed a whole application in VB.NET and all my programmers were comfortable with it and my clients were aware that VB.NET is what we've used-- but they've heard VB.NET is out of favor, and those programmers are now hard to find, etc, etc - you know the drill - perception is as important as reality - we've been down this road with VFP.
And the other part that you're probably aware of is that many developers are religious about their tools. They simply don't want to learn 2 dialects and if they've chosen one they won't change gladly no matter how much you say it's not that difficult to use both.
So, yes, in theory it's simple to switch between the two since, after all they both use the ,NET framework. In reality it rarely works out that way..

>Not sure what your point is. .NET is the "language". C# or VB are dialects. Switching between them is not very difficult. VFP is a completely different paradigm from .NET and its languages. Learning curve there was steep. I began my .NET with VB and now that I've decided to go with EF and WPF C# just seems to have more examples etc i don't have to translate so I switched to C#. Once you know .NET, the transition is not difficult - especially with the kind of hand-holding you get from intellisense.