I need advice about hot gluing plastic and glass. Please see below.?

Did either of the surfaces have any oil (even from fingers) or debris or dampness on them before attaching?

Were the ones that didn't come apart more dimensional/ltextured/etc than the ones that came apart?

Did you perhaps use a high-temp glue gun for the ones that stayed together rather than the more common low temp one? Or did you attach the parts for those less quickly than the other ones?

Did the ones that came apart get more stress than the others (particularly from the side stress)?

You could also try abrading the plastic surface to get more "tooth" and areas for the glue to get down around and grab onto (sandpaper, files, etc).

Instead of hot glue, you could use stronger glues that dry clear though, like 2-part epoxy glues or E6000.

E6000 comes in a large tube and will be clear once set if it's thin (so press parts together firmly and wipe away any excess). There are other tube glues that are intended specifically for smooth surfaces like glass, plastic, etc, so you could look for that prominently on labels on the tubes (at hardware stores).

Epoxy glues come in two parts (often in a dual plunger). After extruding both onto a surface, they're mixed together well, then used for adhesion. The longer the "setting time" of an epoxy glue, the stronger the bond will be btw. One brand that's especially clear when set is Devcon's 2 Ton 30-minute set (according to a friend who tried a bunch for making jewelry and needed the absolute best clarity) though they all may be clear enough for you (and thickness won't matter for epoxy glues re clarity).

get some sheets of glass to use as trays? rather than the plastic. Or have you gotten some sort of oil even from your hands on the glass or the plastic? The hot glue should stick them together, but yes, I have had similar problems. And do you want them to eventually come apart so the vases can be used otherwise?