I realize this is an old thread, but it seemed to be one of the few that focused on the role of mixed languages (C# and VB.NET) as a contributing factor to this.
I ran a few tests across VS.NET 2010, 2012, 2013, and 2015. My empirical results (not validated by any official Microsoft documentation) is that all 4 support Find All References of VB.NET members consumed by C# (albeit, split into apparently separate result groups), as long as project references were added. However, VS.NET 2010, 2012 and 2013 end up in the "from metadata" view for Go To Definition from the C# code to a VB.NET member. Where as VS.NET 2015 appears to have this cross-language Go To Definition navigation working correctly.
I also confirmed that using Resharper's support for External Sources does allow Go To Definition to work under VS.NET 2012, and that almost motivated me to purchase licenses for Resharper for our current project that has mixed C# and VB.NET projects, but now my plan is just to expedite adopting VS.NET 2015.