Under that plastic cover is the usual thin glass nipple. You can usually crush the glass through the plastic cover using a large pair of side cutters or pliers. Another option is to grab one of the CRT socket pins in a pair of pliers and twist it - as if you are pulling it out of the CRT. Usually (not always though) the pin will came loose and let air in.

The usual precautions apply of course: safety glasses on and wear a long sleeved shirt and leather gloves (I use my gardening gloves).