Glass shards certainly arent fun. But even so, even if ZUN is doing the whole "My vampires are different" thing(Spared you guys the tvtrope link) I would think Remilia and Flandre have the hardend Yokai skin to not worry about shrapnal damage.
Anavrin said: Can't just Flan blast up the bottle?
The giant bottle is made from high strength 4" thick acrylic the same stuff they make submarine view ports from and the cork is made from ballistic rubber reinforced with Kevlar.
Reimu probably stuck some sorta seals on them as well so they can't use their energy attacks to break/burn their way out.