We have a corporate website with a large amount of dynamic business application pages (e.g. Shopping Cart, Helpdesk, Product/Service management, Reporting, etc.) The site was built as an ASP.Net Web Application Project (WAP). Our systems have evolved over the years to use .NET 4.5 and various custom business logic DLLs (written in a mix of C# and VB.NET). However, the site itself is still using VB.NET Web Forms. We now have done a few side projects in MVC 4 using Razor/C#, and we want to use this framework for new pages on the main corporate site going forward. What would be the easiest way to achieve this?

• I found this nice list of steps to integrate MVC 4 into an existing Web Forms app. The problem is that because our existing app is a VB.NET WAP, it compiles into a single DLL, and .NET allows only one language per DLL. The site is way too big for us to contemplate converting it to C# all at once (yes, I've looked at the conversion tools, and they're good, but even 99% accuracy would leave us a huge amount of cleanup work.)
• I thought about converting the existing WAP into a Web Site Project (WSP) which does allow mixing languages and then following the steps above, but after a few pages of Google results, I couldn't find any steps for converting a WAP to WSP. (Plenty of sites offer the reverse steps: converting a WSP to a WAP.)
• Another idea I had was to create a completely separate MVC project, and then somehow squish them together into the same folder structure, where they would share the bin folder but compile to separate DLL's. I have no idea if this is possible, because certain files would collide (e.g. Global.asax, web.config, etc.)
• Finally, I can imagine a compromise solution where we keep all the MVC stuff in its own separate application under a subfolder of the main solution. We already use our own custom session state solution, so it wouldn't be difficult to pass data between the old site to the new pages.

Which of the ideas above do you think makes the most sense for us? Is there another solution that I'm missing?