Dry wall in the bathroom is bowed or pushed out by the vent
pipe inside the wall. There is no way to adjust the position of
the vent pipe. I need to straighten the dry wall to install tile on
the wall. Would adding firring strips to the studs that are set
back help to create a level wall?
Yes, adding furring strips (on,, not across, the studs) will let you make the overall wall more flat. Although I have not seen example, I have the impression that bowing of the wall surface would be more noticeabvle with tiles as opposed to just a painted wall.
You may need furring strips of varying thicknesses depending on the nature of the bowing. For a vertical pipe that sticks out, most likely each strip itself would be of a uniform thickness but if the studs themselves are cureved you may need to use myriads of paired wedges on the studs to achieve a flat wall.
The good conscientious technician or serviceperson will carry extra oils and lubricants in case the new pump did not come with oil or the oil was accidentally spilled, so the service call can be completed without an extra visit.