Vapor recovery system

I've never been trained on what a vapor recovery system actual does, how it's used, or used for. Some of our trailers have them. 2" line ran up too with a valve it looks like then ran down the side of trailer then t's off to the front and back both have ball valves and caps.
Can I use that to vent the trailer while unloading just by opening those ball valves? To avoid climbing on top and opening the manhole.
I've heard yes and no.

 

I think the valve on top is hydraulic and tied into the hand pump for the internal valve.

 

No I dont haul fuel. Just grease, cattle feed, molasses, hazmat loads sometimes. I was just curious about the vapor recovery cause I've never used it or been told how to or if I need to.