You can indeed use vanilla js to build Board Views or Dashboard Widget features.
Load the monday sdk in your js file and use it to manipulate data and listen to data changes.
(you do not need Oauth2 for that).
Sorry I do not have any code to show you but I tried this spring for testing and it worked fine.
Lastly, although it’s not vanilla JS I’d also like to link over to the quick-start apps tutorial we have for React right now. Although we have the SDK linked above, perhaps this would be a useful place to look and see some of our example code.