Vertical grips have their place. They can and do help with vertical recoil of all guns and help with the weight of heavier guns. A vertical grip require more use of the brachioradialis as opposed to just the bicep. In other words, one can stand in the high ready for a longer period of time, than without, typically.
In service, I found that any forward grip, raises the rifle of a point of support such as a wall or vehicle hood or the like, thusly increasing your silhouette projection to another person=not good.
They have their advantages and disadvantages like everything else. I run a vertical grip on one of my 308 semi autos, because it is heavy.....but only that one. Everything else is flat on the bottom, for the above mentioned reasons.
If you are on the fence, go with the cheapest one you can find at a gun show. Many military surplus guys will have one for 10 bucks. US GI Surplus.
Overall, for an AR15, I personally would not use one today, as back when it mattered, I ditched it. Good luck.