In"black hawk down" where an RPG gets a Black Hawk heli.. in that case, the RPG is on a high ground (rooftop). Doesnt have to be a vertical profile.
Those RPGs had plates welded on as a make shift arrangement to deflect some of the back blast coming back at the shooter.
Shooters were present both on rooftops as well as on ground around the crash site.
If the helicopter is receding away , then RPG can be launched at a angle close to horizontal inclination.
If the helicopter is approaching the shooter the angle will increase towards vertical inclination.
The helicopters were target of opportunity , so the shooters were located in and around the crash site on ( ground ) and above ( buildings ) the site.
One helicopter which went down was on a descent , when multiple RPGs were fired and one got lucky in that it managed to hit the helicopter and within 4.5 s to self destruct.
In a vertical inclination flight RPG flight path is very unpredictable and susceptible to moderate winds / cross winds.
You have to be very lucky .