I'd be willing to bet money that it was tagged with the vb.net tag (and the code reformatted...correctly) in order to get proper syntax highlighting for the code sample you included.
The code formatting engine takes prompts from the tags applied to the question, and since you included code in VB.NET, including that tag is the easiest way to make it format the code appropriately.
Of course, that's not the only way to provide syntax formatting hints. You can also also specify a language hint using a comment above the code block. But users who have never been on the Meta site probably don't know about that, and it isn't very discoverable. Beyond that, it's much easier just to apply the tag and move on with your life, especially for a question that doesn't involve heterogenous code samples. I'm sure that's what someone did, and I have absolutely no problems with that behavior.
I'm not really sure why you reacted so strongly to that re-tagging, either.
VB gets fewer eyeballs (I think) and I was quite happy with an answer from any part of the .NET Community. A C# answer would be totally fine. It's really a .NET / reflection question, not a VB one.
This claim is specious. I can't imagine that very many of the .NET developers actively ignore questions tagged with VB.NET, and if they do, well then they're language elitists whom you probably don't want answering your questions anyway. They'll look down on you just because the code sample you chose at random to include happened to be in VB.NET rather than C#. And of course a C# answer would be fine; that much is obvious by reading the question.
There's a little mini-war going on in the community regarding the application of language-specific tags (like vb.net or even c#, which is done for the opposite reason, to increase eyeballs on the post) to questions that are actually just about the .NET Framework and/or the BCL in general. I've written about this before here, at least once.
And despite my temptation as a
detail-oriented person pedant to side with the purists on this one, my reasoned conclusion every time has been that it simply doesn't matter.
Yes, you're obviously free to roll back the edits if you want. But don't get into an editing war with someone about it, don't have a pointless discussion in the comments about it, and don't edit meta-text into your question justifying your choice of tags. It just doesn't matter enough to justify the clutter.