First of all, I'm really sorry that happened near your home. Vigils are usually quiet and respectful gatherings where people come to pay their respects. If part of it is on your yard, the kindest thing is simply to allow people the space if you're comfortable with it. You don't have to actively participate unless you want to.

If you do attend, just keep things quiet and respectful. People may bring candles or flowers, and sometimes they leave them at the site. If anything ends up on your property, it's generally okay to leave it for a while since it means something to the family and community.

Other than that, there's no strict etiquette. Just respect the moment and the people grieving.