You need to place components written in different languages in different subfolders.
If you create two subfolders in the App_Code folder named VBCode and CSCode
Then you can use the web configuration file to use components written in both VB.NET and C#.
File: Web.Config
When the contents of the App_Code folder are compiled, two assemblies are created.
one that corresponds to the VBCode folder and one that corresponds to the CSCode folder.