Turns out if I remove the custom Vector Tile Basemap that I created and use in the web map that feeds the web app, it will work as embedded in IE 11. Some combination of being embedded and not being able to render Vector Tiles...the web app works fine in IE 11 (with Vector Tiles) as a stand-alone (not embedded). But once in embedded, it won't render the web map ( bc of Vector Tiles). Also could be due to the fact that we use SharePoint (or so IT tells me). My guess is that the Vector tiles file is too big too render in IE?