I'd be interested in how on earth it kept working without a vacuum.
A vacuum isn't necessary, provided there is no oxygen inside the glass then the filament cannot burn. Halogen being one of the many inert gases that can be used to fill the glass and protect the filament.
In the distant past some trickery was used in education of students. The lamp may have been manufactured with water and an inert gas in glass as a "trick" question to encourage the students to think out of the box.