I hope you meant vb.net and not vb. Or did you mean vb script?
If you meant vb.net and c#, which I did about a good number of years ago, then you can do this via the app_code folder.
Create subfolders - one for c# and another for vb.net. Place respective classes in the folders.
And then ensure that you configure these folders in the web.config's codesubdirectories node.
<configuration>
<system.web>
<compilation>
<codeSubDirectories>
<add directoryName="dirThatContainsVB"/>
<add directoryName="dirThatContainsCsharp"/>
</codeSubDirectories>
</compilation>
</system.web>
</configuration>
Web.config rules apply as before.