Why aren't tilting propellers used as an alternative for ailerons or elevators? I was wondering what are the reasons for not using tilted propellers fixed to ailerons/elevators/elevons? Or even on wings with no control surfaces at all? Wouldn't the 2nd way of thrust vectoring greatly simplify aircraft design?

There is a huge variety of VTOL aircrafts, but all seem to have puller propellers in frontal wing area with ailerons/elevators separate.
It isn't done because a moving control surfaces is easier to design and build than an engine mount that rotates. 

Plus the associated structure needed to accommodate the thrust, p-factor, and gyroscopic loads is going to be a lot heavier than ailerons, spoilers, moving tail plane, etc.

Another issue is if one loses one or both engines, your roll and pitch control disappear as well.