if it is a known system .h then no rescan should be required. for local .h files, i jump to the file (alt_g on the #include works nicely ) and then manually reparse the .h when this happens to me, and i am in a hurry.
it is an #include "xxx.h" or a relative path #include "lib1/xxx.h"
The file (or lib1 subdirectory) is located in e.g. d:\\sources\\include
There is an environment variable KA_EXT_INCL pointing to this directory (KA_EXT_INCL=d:\\sources\\Include), and the project settings have $(KA_EXT_INCL) as "additional include path"
..but jumping to the include, and reparsing it would be fine (Not sure if alt-g always finds the file in this scenario, the VC6 "open document "xxx.h" command works only after VC6 recalculated it's dependencies.)
i know in the past VAX has had problems with environment variables in include paths. if this path is basically constant you could try adding it to VAX's directory's list to see if that helps