Q: can vanilla windows talk to each other? A: yes they can!

vanilla-move-layers-test.py

from AppKit import NSApp
from vanilla import *

class LayersWindow(object):    

    def __init__(self):
        self.w = FloatingWindow((123, 200), title='layers')
        self.w.layers = List((10, 10, -10, -10), ['foreground', 'background', 'sketches'])
        self.w.bind("close", self.closeCallback)
        self.w.vanillaWrapper = self
        self.w.open()

    def closeCallback(self, sender):
        print("close and delete")
        del self.w.vanillaWrapper
        
class MoveWindow(object):

    def __init__(self):
        self.w = FloatingWindow((123, 80), title='move')    
        self.w.value = EditText((10, 10, -10, 20), '10')
        self.w.apply = Button((10, 40, -10, 20), title='apply', callback=self.moveGlyphsCallback)
        self.w.open()
        
    @property
    def selectedLayers(self):
        app = NSApp()
        layers = []
        selection = []
        for window in app.windows():
            if window.title() == 'layers':
                delegate = window.delegate()
                if hasattr(delegate, "vanillaWrapper"):
                    vanillaWrapper = delegate.vanillaWrapper
                    layers.extend(vanillaWrapper.w.layers.get())
                    selection.extend(vanillaWrapper.w.layers.getSelection())
        return [L for i, L in enumerate(layers) if i in selection]
        
    def moveGlyphsCallback(self, sender):
        value = int(self.w.value.get())
        f = CurrentFont()
        for glyphName in f.selectedGlyphNames:
            for layerName in self.selectedLayers:
                g = f[glyphName].getLayer(layerName)
                g.moveBy((value, value))
            
LayersWindow()
MoveWindow()