So I have a ASP .NET vNext project in VS 14 CTP. I am targeting .NET Framework 4.5.1. I added a reference to a NuGet package, which does not have a build specific for vNext.
According to ASP.NET improvements in Visual Studio 2015 CTP 5, you will be able to reference standard projects from a vNext project. You can do this by simply using 'Add Reference' menu with right click, or manually adding it to the project.json under dependecies.
Just avoid making cycling references (A references B and B references A) since that will cause Visual Studio to auto refresh the focus of your window on every 5 seconds.