Insulation, Radiant and Vapor Barriers - Ceiling vapor barrier?

I removed a 4’x9’ section of ceiling drywall in my dining room to do some repairs. Now it’s time to put up new drywall but I noticed that the existing ½” drywall on the rest of the ceiling has an aluminum foil backing, which I assume is a vapor barrier. However, none of big box stores carry drywall with an integral vapor barrier.

So how is the ceiling vapor barrier done these days? Or have vapor barriers on ceilings fallen out of favor?

Other facts:
I live in central California (dry climate and mild temperatures).
House is low budget tract home built in late 50’s
Attic is vented in a rather traditional way, by round holes along eaves (holes in the frieze blocks), 3 long narrow ridge vents and a circular 1 ft hole where one of those circular vents that passively spin is mounted.
No air conditioning in house.
No unusual sources of moisture in house (just the standard showers and gas stove/oven-unvented). No signs of mold in attic.
Existing attic insulation is R-11 craft faced bats. I intend to add some more insulation, either more batts or blown cellulose.

Although the foil does act as a vapor barrier, it was probably intended as a radiant barrier. The vb is intended for the warm side of the insulation, which in ca should be above the insulation. But since you do not insulate and since vb's are not always necessary, I wouldn't worry about it. Air sealing if far more important as humid air will transport moisture much faster than working it's wat through the drywall. Here's some reading:
Energy Savers: Vapor Barriers or Vapor Diffusion Retarders (http://www.energysavers.gov/your_home/insulation_airsealing/index.cfm/mytopic=11810)

Bud