Correct. While you can manually upgrade from .NET Framework to .NET Core, you cannot simply change the version inside the project and have it update. There were too many fundamental changes.
Unfortunately, the project you want is the ASP.NET Core Web API, and Microsoft has decided that VB.NET is not the way of the future, so it is unsupported. See here:
https://learn.microsoft.com/en-us/an...re-api-project
The recommended solution is to basically create an N-Tier solution, e.g.
- API Layer (C#)
- Domain Layer (VB.NET)
- Infrastructure Layer (VB.NET)
Keep the API layer as lean as possible and have the domain/infrastructure layers as class library projects that hold your business logic and what have you.