I have a similar issue with tkinter on a Windows VS Code / PIO install. However, installing “tk” via pip is not sufficient.
I had to set the VSCode preference platformio-ide.useBuiltinPython to false and thereby make it use my global python installation, which had tkinter included (it is a python installer option that you actively had to uncheck during installing python). An alternative to all that could be to install ActiveState as a standalone Tkinter installation for windows, I guess? (see TkDocs Tutorial - Installing Tk )