Why is this "sad"? VB.NET offers little (anything?) over C#. I've never seen a bit of VB.NET you couldn't transpile into perfectly readable C# outside of really gross and bad implicit type conversions around Booleans.
If Xamarin should care about VB.NET, it should be better at things than C# is.
About the only thing I'm aware of that VB.Net has that I really like is an XML Literal syntax that seems to have been inspired by E4X ... that's about it, C# has adopted all the other features that VB had over it. And with LINQ to XML, it's even more limited in use.
It offers the ability to run VB.NET codebases. Transpiling them into C# offers cost with little benefit (for stable code that is rarely updated), while running it on Mono allows for hosting infrastructure upgrades.
But if anyone cared, they would do it. No one cares, so it does not get done.
If Xamarin should care about VB.NET, it should be better at things than C# is.