1. To unlock the Character Vision you must complete the level ULTRON UNDONE.
2. This will automatically unlock the character Vision fast travel to any free play location e.g Manhattan.
3. Now press and hold
scroll to the bottom row, the fifth icon across the pink looking character Vision press select.
4. Now switch to your second character by tapping
then repeat process 3 above, once you press select.
POP achievement unlocked!
Congratulations and happy hunting