I've done some stuff where I made the GUI in CSharp and the actual scripting in JScript. Too bad there's no real JScript support in Visual Studio.NET. I'm too lazy to convert the form designer generated code to JScript, although it's not that hard.
You can import the Sonic Foundry assembly into a CSharp project, but that doesn't expose the functions in the same way.
I've been able to use the VegasCOMClass from C# to do some minor stuff (i.e. launch V4 and print the version number), but the interface isn't the same as it is in the script engine - presumably the script engine initializes some globals before running the script so that you have access to a "Vegas" object that you don't get with the COM class.