iPhone displays are a little touchy and it is easy to accidentally bump the keypad and switch to a different screen or even a different app. Luckily with the iPhone settings you can easily lock and unlock your display on any screen you want.
On your phone tap Settings >General>Accessibility>Guided Access, and tap to turn Guided Access on. This feature allows you to lock and unlock an app by tapping the Home button three times. It also allows you to disable other functions on that screen so you can disable the left hand menu bar. With that disabled you won't be able to navigate off of the current screen accidentally until you tap the home button three times.
Once you have it turned on start your Ventus app and go to the screen you
want to lock and tap the Home button three times. The first time you do this
or any time you exit Guided Access, is it will give you the option to circle
areas on the screen you want to disable. Use your finger to draw around the
entire left hand menu bar. Don't worry if you don't get it exactly where you
want because a box will appear where you have drawn that you can drag to the
size and location you want. Tap End and set up a 4 digit passcode. Pick
something simple (I use 1234).
That's it. Now when you use Ventus, or any other app, you can go to any screen and simply tap the Home button 3 times and your display will be locked on that screen until you unlock it.