Body armour dissipates lots of the bullet energy in the act of breaking. Prince Rupert drop armour would mostly result in the bullet snookering a glass chunk into you instead. Even if you could make a solid glass plate that wouldn't break when shot (doubtful), the transmitted energy of a bullet would still be more damaging because it would all end up in your body, not in heating up the broken shards of a ceramic plate.
The transmitted energy does end in your body in case of normal bulletproof vest tho. Only very little is converted into heat, the main job is to spread it around. Movies as usual do a terrible job of portraying reality here.
Ceramics aren't used because they break on impact, they are used because they are harder per kg than steel so the whole thing can be lighter.
Imaginary "unbreakable plate" would still be backed by layers designed to slow down the pulse and dissipate that energy and if it was thinner and lighter than ceramics that means you can put more materials behind it and still come out ahead
Only the head of the Prince Rupert drop is tough, due to high levels of stress in the glass (its made by dripping viscous melted glass into water). Interestingly, the whole drop with shatter into glass dust if the filament like tail is broken!