VB.NET and C# both are integral part of the .NET framework. Both of the languages have a lot of similarities in language constructs and language design with minor differences in the syntax. C# is more like C++ and Java in its syntax while VB.NET lends its syntax from VB6 a great deal; although VB.NET can not be seen as the successor of Visual Basic at the level of overall language design and the vision of its creators.
The general conception is that most of the VB6 developers will upgrade to VB.NET while developers coming from Java or C++ to .NET are likely to go for C#. Since web developers for Windows using ASP are familiar with VB and VB Scripts, therefore, most of the web development with ASP.NET at the start is likely to be dominated by VB.NET; but after sometime C# will be able to attract at least half of these developers. Keeping technical side away, C++ and thus C# developers are generally paid more than those of VB developers. So, this might also be the factor for making C# the language of choice on .NET framework.