Only disadvantage I can see is extra bit of weight. But what are the advantages?

I've always used the C clamp grip on my rifles. Thought about getting a weaponlight installed and it seems most guns pictured with weaponlights all have a vertical grip. Some have a pressure pad taped to the vertical grip or the vertical grip is positioned near the light at the front of the rifle. Other times I see the vertical grip far back near the magwell.

Is there a "proper" way to run one? I'm tempted to just spend the $20-$25 to get one for trial/error, but I'd like to at least know the advantages to one.

"Proper" is what works best for you. In my case I prefer the handling characteristics of a full-size vfg w/my support-side thumb activating my WMLx, I find it less fatiguing to shoot/carry:

Tomac

for me the advantage is comfort. although i prefer an angled grip to a regular vertical.  but holding  the rifle by the hand guard is a bit rough on my wrist.

i'd say proper is anything but a mag well grip because if the gun blows, good luck. although i will admit, i used to carry it like that, and it was comfortable.  but after seeing threads on here, no more.

like most things, it's all about what works best for you. I prefer VFG's, can't stand AFG's and can run without either if needed. My left wrist has taken some damage over the years and a VFG allows me to run without discomfort.

There is no right or wrong approach, it's all about you and what allows you to shoot the best.