Soon i will be going on a trip to namibia and do not want to take my laptop with me( anyway i will often not have wifi anyway there) therefore have no assistant 2 when problem occurs.
The question is: when does the calibrate vision sensor warning pops up? Is it on a regular base or after a certain amount of flights ? And pressume it pops up in the middle of nowhere , can i still fly the mavic with shuttingdown the visionsensors?

You can calibrate the IMU's, Camera Gimbal & Compass right from the Go4 App, The VPS (Visual Positioning Sensors) need a computer to calibrate which in my experience is very rarely needed, just try not to bang the thing around a lot, bring it as carry on if you are flying.

after 1000km mavic flight i never had a calibration notice pop up. yes mavic still flies perfectly without vision sensors, pilot must just pay good attention when flying close to anything. Rather be a bit too far away than too close

When there is an error on the visual system, the app will tell you to calibrate the sensor. You can shut down the forward and downward sensor to fly, but to ensure the flight safety, we suggest flying when all function works properly.