Choosing a Code Editor
vvvv doesn't include an editor for editing C# code. Instead you can choose your own. Here are some suggestions:
Visual Studio
Visual Studio >= 2022
During installation make sure to check the ".NET desktop development" workload.
Visual Studio Code
After installation make sure to install the "C# Dev Kit" extension.
JetBrains Rider
Text Editors
If you don't have any of the above editors at hand, really any text-editor would do to simply edit the C# code. Obviously you'll be missing out on some convenience, but for simple scenarios this may be enough.