Yes, it's a vapor barrier. Where do you live; is the attic properly vented; is the house air conditioned; are there any unusual sources of moisture in the house?

I live in central California (dry climate and mild temperatures).
Attic is vented in a rather traditional way, by round holes at eave (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. No signs of mold in attic.

As moisture source, I forgot to mention unvented kitchen gas stove. But I guess that may not be unusual...

I don't see why you would need a vapor barrier at the ceiling in your climate. Be sure no exhaust fans terminate up there. The original foil-backed board was possibly a radiant-barrier if no insulation was covering it.

Thanks for the suggestions,

This was a small area (9Âx4Â) so, for now, since I needed to proceed, I decided to staple a 6mil clear plastic to the joists, just in case. I figured that if it turns out that it will be doing more harm than good, I can always rip it from the attic side.

I did though already find a drawback. The vinyl prevented me from gluing the ceiling drywall to the joists (I had originally intended to use both glue and fasteners). I did though use drywall screws to secure the drywall, so IÂm not too worried.