First of all, is this project being done under a permit?
The only way you'll know if you can move that pipe w/o some structural work is to open up the ceiling below (if it's on the 2nd story) or the floor between the toilet and the pony wall. I would choose the ceiling, because your access to the toilet waste and vent will probably be better. You may even have to open a wall below. That pony wall removal is getting more expensive with every word I type. Why do you want to remove the pony wall? Are you interested in making your vanity top longer? By the looks of things, the pony wall (with the vent in it) could be moved over maybe 9" to the right without getting too close to the wc. That still would require opening the ceiling or floor to shorten the vent. If you are really lucky, opening the ceiling may reveal an ability to move the vent into the wall behind the toilet and vanity below floor. Since this wasn't done in the first place, there is probably something structural, like a floor joist, in the way.
Maybe you should hire a plumber to do the thinking and the dirty work.