make VAR stay focused

Yes, you can pull down notifications and press on @Voice notification, or if you have @Voice icon on Android home screen, you can press it too. Or press the app switcher button at the very bottom of Android screen (some devices have it at bottom-left, others at bottom-right) and scroll the screen horizontally, find @Voice screen and press it...
 
Note that "Read aloud" item on the menu that appears, after you select text in any app, has a limit of only about 500 characters. It's a limitation that Google imposed on the functions added by other apps to this so called "context menu", and there is no way for apps to increase this limit. If you need to get more text into @Voice app, simply select and press "Copy" button, then pull down notifications and press the round button at right in @Voice notification.

Currently there is no way to make the app come to front after you press "Read aloud" in the text selection menu in any other app. Instead, after selecting text, press "Share" in the toolbar that appears, then press @Voice icon. Then @Voice app will come up to front and load the selected text, read it aloud. Also, this method has no 500 characters limitation as "Read aloud" function on the share menu has, so probably it's the best option.

I just want you to know that that 500 char limit doesn't seem to be present when I do a select all on a particular page I visit daily. I've tried other random pages as well and it too doesn't seem to be restricted.