WEIGHTED VEST

I'm not a runner, but...

I use a non-adjustable (weight) 15 lb weight vest for walking: I walk 2.5 - 3.0 miles about 5x per week, and I really like using it.

I would imagine that, on the one hand, a weight adjustable vest may be beneficial. Having said that, my concern for runners would be the bulk/comfort factor (breathability, etc.) of using it. Of course, that can vary by distance I would imagine. Those are two concerns that aren't really issues for short walks.

I'd be curious what runners have to say, but those are some things to think about.

I would also be concerned about the extra stress it puts on your joints.

What's the thinking behind doing this?
Unless you are actually training to run with additional weight, I would think there are other ways you could increase the training load that would be better for your joints.

Depending on what you are trying to achieve, you could also just consider:
running further
running faster
a weighted sled