Is SCons compatible with make?

No. The SCons input files are Python scripts, with function calls to specify what you want built,

Is there a Makefile-to-SCons or SCons-to-Makefile converter?

There are no current plans for a converter. The SCons architecture, however, leaves open the future possibility of wrapping a Makefile interpreter around the SCons internal build engine, to provide an alternate Make-like interface. Contact the SCons developers if this is something you're interested in helping build.

Note that a proof-of-concept for parsing Makefiles in a scripting language exists in the Perl-based implementation for Gary Holt's Make++ tool, which has its home page at http://makepp.sourceforge.net/

Does SCons support building in parallel, like make's -j option?

Yes, SCons is designed from the ground up to support a -j option for parallel builds.

Does SCons support something like VPATH in make?

Yes. SCons supports a Repository() method and a -Y command-line option that provide very similar functionality to VPATH, although without some inconsistencies that make VPATH somewhat difficult to use. These features are directly modeled on (read: stolen from) the corresponding features in the Cons tool.