A single layer of glass on a flat surface is too elastic to control any panel resonance. It's great for non-flat surfaces if you can build up a few layers, but otherwise won't do much.
The pressure differential between the compression and rarefaction swings in a vented enclosure is higher than that of a sealed enclosure, so typically a vented enclosure will need to be stronger to hold up to those oscillations. That being said, 5/8" is perfectly acceptable for most enclosures that are less than a few cubic feet, especially with low-power situations. Bracing can really
reduce the need for thick walls; if two parallel sides of an enclosure that measure 1/2" thick have a perpendicular brace running between them, it increases the stiffness to about the same level as if they were 1" thick material with no brace. So if you want to use 5/8" but add a few braces it should be a really strong box.