This is an article that outlines the best practices for using VB6 and VB.NET code in the same project, albeit not in the same IDE. It outlines how to build "COM Callable Wrappers" to allow communication between the VB6 and .NET code and has sample code. This has been a question that has been asked of me for as long as .NET has existed, and it's great that Scott Swigart wrote a nice little article about it.
Here's the article link: http://msdn2.microsoft.com/en-us/library/ms364069(vs.80).aspx
~ Robert Shelton