I started using VB6, then I switched to C++ because people were calling VB6 names, now I've switched entirely to C#.
Language doesn't matter. Improving programming involves things that apply to all languages. That said, don't use VB6 as it doesn't have some useful OO features and being an old language the documentation, examples and tutorials online are sparse.
The easiest switch you can make, if you really want to leave VB.Net, is to C# as the two are very tightly linked. You'll have to learn new syntax, curly brackets are probably the biggest difference, but that's about it. There are sites with chart comparisons between VB.Net and C# syntax:
http://www.harding.edu/fmccown/vbnet_csharp_comparison.html.
Java is similar to C# so getting used to C# should help any adoption of Java.
I wouldn't bother learning C++ unless you are interested in understanding low level memory management and pointers. It's useful to know, but that could be said about a lot of things. A few things I would really recommend are:
1a) Read other people's code that is well known for being written well for inspiration. Don't bother to understand how the actual program works, that takes ages, just read the code to see how they've structured and modularized the source code. Just seeing how things had been broken down into functions and split into different files.
1b) Once you realize how to structure code neatly one big trap is to spend too long doing that and to become unproductive. Strike a balance between spending ages doing things perfectly and being productive. Better to write things untidy and refactor later.
2) Learn OO concepts like inheritance and polymorphism. They aren't necessary, but they can help modularization a lot.
3) Improve at tracking down bugs and fixing them. Aside from using tools like the debugger to help, this really involves grinding on ever occurring bugs to increase XP. Bugs will always happen so getting a routine to hunt their source can be a real time saver. As a bonus 1a) should often help to reduce bugs or make them easier to track down.