これは(Debianの9上にワニス .0.0インストールから)VERSIONING下マニュアルページVCL(7)で説明されている:
Multiple versions of the VCL syntax can coexist within certain
constraints.
The VCL syntax version at the start of VCL file specified with
''-f'' sets the hard limit that cannot be exceeded anywhere, and
it selects the appropriate version of the builtin VCL.
That means that you can never include "vcl 9.1;" from "vcl
8.7;", but the opposite may be possible, to the extent the com‐
piler supports it.
Files pulled in via include do not need to have a "vcl X.Y;" but
it may be a good idea to do it anyway, to not have surprises in
the future. The syntax version set in an included file only
applies to that file and any files it includes - unless these
set their own VCL syntax version.
The version of Varnish this file belongs to supports **syntax 4.0**
only.