In a lab, you get a water tight seal using all ground joints. I doubt they use very many clear glass joints. Not sure about “vapor”, but I’d guess the performance is similar. If have two perfectly formed and polished glass joints, clear, you can get a good vacuum type seal, but you rarely see perfection in the glass we buy….
When I high pressure clean using adapter from sink to ground joint via silicone tube, the joint won’t break free and squirt water all over, but if one of those two joints were clear, I’d have a gusher…