Thanks for asking, mika.

Yes, I get the same refresh figure; no, vesafb.txt cannot help; no, fbset cannot
help.  Fbset will not change the fb mode because vesafb driver only supports
VESA BIOS Extension 2.0 not VBE 3.0.

Only grml can change the vesafb driver mode(s).  They are compiled-in, AFAIK,
with vesafb driver.  (Maybe a .h file has the timing values?  The grml mode is
not listed in /etc/fb.modes.)

VESA standards are not secret, but they cost money.
http://www.vesa.org/Standards/summary/2004_10.htm
http://www.vesa.org/Standards/summaries.htm

"Display Monitor Timings (DMT) are a list of VESA standard pre-defined timings
which are commonly used within the Computer industry."
http://www.nvidia.com/object/advanced_timings.html
Manufacturers standardize on them.  They even advertise compatibility with VESA
timings.
http://support.dell.com/support/edocs/video/27vnj/ge/specs.htm
(Auflösungen)

Above ~75 Hz the DMT no longer apply (?) and then it's Generalized Timing
Formula (GTF).  (I am not sure about differences.)

We miss ~3 console lines in vesafb, at top of screen.  It's very annoying.  X11
still works because we can set timing mode in xorg.conf.  We can't change fb
mode for vesafb driver.  We can't use another fb driver, because of nVidia
conflicts.

Existing infrastructure - hmm, in this case, the word "decrepit" might fit. 
Like an old bridge.  The vesafb driver is growing old, and not many Linux people
look at the code any more.  That is my impression.  But everybody uses it,
especially text users.

A clue to "decrepit" is the lacking error when you change fb mode.  Since vesafb
can't change mode, it should report an error, or fbset should.

This is the extent of my knowledge.  I do not have VESA documents.  Someone at
Debian should have a copy.