VB .Net such as VB 6 are interpreted languages, but a package of runtime modules are required to run applications. VB .NET applications needs a .NET Framework to run on your PC and making DLLs for mIRC with these languages I think is impossible... I had built some DLLs for mIRC in C++, if you would develop with Visual Studio .NET you can create a "Win32 Project" from the project wizard, create a source file and if you would an header file and insert some function...
Note: you MUST include the standard entry point function to make mIRC to recognize the DLL... Take a look at the "DLL Support" chapter in mIRC help file for more informations...
Have Fun...
Paride.