There is a fantastic 2010 blogpost[1] from the Visual Studio Languages Product Manager that explains the rationale for supporting VB.NET as well as C#, as well as the strategy for doing so.

Of interest: he states that both languages have (had?) "roughly equal adoption".

[1] http://blogs.msdn.com/b/scottwil/archive/2010/03/09/vb-and-c...