I read this question when I noticed a curious comment below:
not sure what the question is: you can use the VB.NET and C # projects in one solution (although I would not recommend this).
I do this very little, since we have the old VB.Net code, and the new code is written in C #. Is this really not recommended? Why not?
There is no real reason to avoid this other than adding complexity from having two languages in one “solution”.
Your scenario (working with an outdated product, but adding new features) is a good reason to use both languages in one solution.
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Why do you need this? If you have outdated code that you want to use, you save this code in your own components, you do not mix it with the new code. This is not recommended because it does not contribute to the Clean Code. This can lead to the fact that you have a solution that is difficult to read and maintain.