Vendor branches and adding files subsequently

[Frank A. Uepping]

Hi,
look at this session:

# Some initial import:
cvs im -m "blahblah" foo them foo_1_0

# Check out foo and alter some files:
cvs co foo
edit...
cvs ci -m "blahblah"

# Oops have forgotten a file in my initial import.
# Lets add the file subsequently to the vendor branch instead of doing
# a complete new import (because I am lazy ;)
cvs up -r them
copy .../bar.c .

cvs ad -r 1.1.1 bar.c
cvs commit: cannot add file `bar.c' with revision `1.1.1'; must be on trunk
cvs [commit aborted]: correct above errors first!
# Doesn't work, why (them is branch:1.1.1)?

cvs ad -r them bar.c
# Creates a new branch 1.1.2 with bar.c as the only file.

Apparently I can't add files to a vendor branch subsequently, is that true?
What is with `-r them' variant; is the automatic creation of the branch
1.1.2 a defined feature or a bug?

/FAU