Vent Height

This is a very good question and I thank you for asking it.
When we consider "Vents" we must acknowledge that they come from different sources and will have different kinds of discharges. Some dangerous and some just plain annoying. People (Operators and maintenance workers) need to be protected from unpleasant or deadly discharges. Likewise, buildings, equipment, structures and piping should also be free from direct vent pipe discharges that could cause damage.
So, I suggest the following:
1. Is there a Code (where you or your project is) that applies to this?
2. The open end of the vent pipe should be a minimum of 8'-0" (2.4m) above Grade or Platform associated with this vent. The reason for this is Safety. Most discharges from vents can be very hot, toxic or harmful in some manner. If the open end is low enough such that a person could reach it with their hand then, sure enough they will do just that. What will happen to the unthinking person who places a hand over the vent pipe and just then there is a release of super-heated steam? No hand, right? What if it is low enough for the person to look down the pipe? That would be worse.

As a piping designer you should "look" around your design to see what could be affected by the released vapors.
a. People first - Is the open end out of reach?
b. Equipment second - Picture an inverted 45 degree cone sitting on the vent pipe opening and check in 360 degrees in all directions (the wind does not always blow where you want it to) to see where the wind might blow the release.