Enable VoiceOver ine only one app?
You can't enable VoiceOver for just one app, but you can easily turn VoiceOver on and off.

Method 1(Siri): Just launch Siri(by pressing and holding the home button), and say "Turn on Voice Over", and and Siri will reply(no reading necessary). You can also turn Voice Over off, by saying "Turn of Voice Over" when you leave the app.



Method 2(Acessibility Shortcut): Go to Settings > General > Accessibility > Accessibility Shortcut and tap "VoiceOver". Now whenever you triple-click the home button, it will enable voice over, and when you triple click it again, it will disable VoiceOver.