I recently took the plunge. I had been using e-TextEditor for everything but .NET, and VS for .NET. Now I'm using gVim for the majority of my editing for all languages I am working in. However, I still use Visual Studio with ReSharper for certain things: debugging, intellisense (only when I'm stuck; I'm trying not to rely on it much. you can also use vim plugins or create your own to let you quickly jump to MSDN and find something), and editing project/solution configuration.
Alternatively, you could use ViEmu (emulator plugin for VS, amongst other MS things), but I like the split buffers so I haven't gone down that path.
In summary, I don't think it's an all or nothing deal. You should use the best tools for the job. I think using Vim will make you more productive for plenty, but not all tasks.