I don't know either, but when I tried flfmt with a vac score as an experiment, I got this:

ole@ole-TECRA-R940 ~/Desktop/plan9 $ bin/fossil/flfmt -h 192.168.0.101 -v
f648dbae0075eb73bc394ad6cd4c059e655e127c fossil.dat
fs header block already exists; are you sure? [y/n]: y
fs file is mounted via devmnt (is not a kernel device); are you sure?
[y/n]: y
0xfb1e734c
0x1d1feaf1
c85978546e4048fce83120d3992cfc2f57ff2f8c
bin/fossil/flfmt: bad root: no qidSpace

    /*
     * Maximum qid is recorded in root's msource, entry #2 (conveniently in
e).
     */
    ventiRead(e.score, VtDataType);
    if(!mbUnpack(&mb, buf, bsize))
        sysfatal("bad root: mbUnpack");
    meUnpack(&me, &mb, 0);
    if(!deUnpack(&de, &me))
        sysfatal("bad root: dirUnpack");
    if(!de.qidSpace)
        sysfatal("bad root: no qidSpace");
    qid = de.qidMax;

It seems that the vac archive does not contain the max qid that
flfmt needs. This seems strange to me, as vac -a should need this info just
as much as fossil needs it. Maybe it's tucked away somewhere else. Guess I
need to look some more at the code.

Digging further, I found the comment in file.c, but did not pursue the matter:

 * Fossil generates slightly different vac files, due to a now
 * impossible-to-change bug, which contain a VtEntry
 * for just one venti file, that itself contains the expected
 * three directory entries.  Sigh.
 */
VacFile*
_vacfileroot(VacFs *fs, VtFile *r)