I have just switched from Java-development (where I used IntelliJ) to an old WinForms application in VB.net. I have decided to use Rider, since I really liked IntelliJ.
From what I understand Raider's WinForm designer does not yet support VB.net-WinForms.
So my question is if there is a plugin that supports WinForms designing in Raider (to avoid having to switch to VS and avoid using 2 different IDEs).
The Winforms designer from Rider does only support C# with WinForms under Windows. It has recently added support for other toolkits, maybe in a cross-platform manner.
The answer to your question is no: and I don't think they are going to include since there is probably not enough demand. You can still use Visual Studio for the designer and go back to Rider when that part is finished.
My personal advice is to avoid use any designer. WinForms is really easy to grasp, and there are tutorials out there that make the task even easier.
One possibility would therefore be to get rid of the designer files and manually modify the code. An important outcome is that the project will be then editable in any IDE, not only Visual Studio or others with compatible plugins.