According to Jason :
When I put in the first bat of R38 I checked and there is no vaper barrier
anywhere. It goes something like this: ceiling wallboard (1/2 inch thick),
ceiling joists (2x4), 2 -3 inches of probably rockwool insulation (at least
35 years old) between the joists, R38 bats of insulation laid perpendicular
to the joists to cover them.

The first layer is unfaced and I was planning on purchasing unfaced again.
Is the lack of a vaper barrier a problem?

A good paint job on the ceiling below acts as a vapor barrier.
Especially with a "vapor barrier" rated paint. Ask about it
at a good paint store. HD _might_ carry such.

R38 is nothing to sneeze at. Going to R76 would be overkill methinks,
and not going to make much difference over something slightly more
modest. I have seen a modern house done to R70. Almost scary ;-)

If you're doing this for a sale, it might be worth it. If anybody
notices.

A single layer of 3.5" (R16) would be easy, cheap, and get you to R54.

By the time you're at that level, other issues will end up being
the biggest losses.

A house that started with 3-4" of rockwool (about R16.5 with
modern rockwool, probably somewhat less with older stuff) is going
to need attention to other things before going to R76.
--
Chris Lewis, Una confibula non set est
It's not just anyone who gets a Starship Cruiser class named after them.