The small glass "bulb" (more frequently called 'nipple') is used in the manufacturing process of the display as the vacuum is being drawn through it. Once the proper level of evacuation is reached it is melted shut.
So when the nipple breaks off, the display is again filled with air and won't work. It has to be replaced. According to earlier posts, Noritake who is the manufacturer don't make it anymore (unless they get a 100+ or so order) , stocks seems to depleted. I don't recall what the spare part situation is for the display at the official support, so contact them to find out. If you can find the display elsewhere, the actual replacement procedure isn't very difficult if you have decent soldering skills.
Part number for the display has been posted on the board, so do a search. Same for more discussions about the perils of broken nipples
/Michael