Vintage MTB, mounting new v brakes
I find the v-brake on the front fork, which often times has just one hole for the spring pin, is a smoother install. Idk why.  There is no difference in "front" or "rear" v-brakes except the direction of the pads.

Typically at the rear, the frame's tabs where the brake bosses are threaded or brazed on will have three holes per side drilled in them from which to choose the insertion of the brake arm's spring pin. Each arm's spring pin should be placed in the same relative hole (if the left spring's pin is in the middle hole, the right spring should also be in that side's middle hole).  Spring tension is highest when the upper most hole is used. Typically the middle hole is chosen. 

One way to ease getting the pin into the desired hole is loosen the brake pad's nut enough so the pad can be rotated parallel with the arm. In this position the whole side of the brake can freely rotate on its boss without the pad bumping into the seat stay.   This way allows easier access and placement of the spring pin into your desired hole.  This may be the crux of the problem. Is the brake pad blocking the movement of the arm before it's pin is sufficiently lined up with the hole?  

Park Tool has a great tutorial on installation of linear pull brakes.  V-brakes is a term coined by Shimano and is a linear pull brake system.