VLT stands for Visible Light Transmission - the amount of light passing through the lens. Think of it like your sunglasses; the darker the tint, the lower the VLT. Most lenses in the snow industry range between 5% for the darkest of smoke lenses and 80% for clear lenses. If you only have one lens, we'd recommend something in the 15-30% range to accommodate for all but the lowest light conditions.
Remember, VLT % has no impact on UV protection. All of our lenses are 100% UV400 rated, so no matter what VLT lens you choose, your eyes will always be protected.