Answering as a comment.

I have a .NET 8 API with VueJs frontend using <SpaRoot> setup.

Firstly, check if you're using the SpaRoot in the csproj of the server or not.

The SpaRoot should be pointing to your front end UI folder and there should be an execute command near that SpaRoot element.

You don't need a multi-app start config. When you run the server on its own it should boot up and display "waiting for SPA".

Then VS will run your launch command, think it's default to npm run dev. Where you'll see a console population and build and run the server-host for your UI on a port.

Your page is displaying at your API port and will redirect to the port your UI is at.

If you get debug port errors, could be a config issue or port mismatch in launch settings json and/or your front-end thing.config.json AND csproj Spa settings.

If all else fails, check port availability incase somthing is using it. It happens sometimes.