Bundling folders with svn copy and vendor branches

I am trying to use vendor branches to manage some third-party add-ins for a
project,
but I'm having difficulty finding a clean way of collating the vendor
branches.
(I'm also having difficulty tying down my explanation without resorting
to show-and-tell, but please bear with me.)

If I have a structure like the folowing:

/vendor/libcomplex/current/
/vendor/libcomplex/1.0/
/vendor/libcomplex/1.1/
/vendor/libgeometry/current/
/calc/trunk/add-ins/
/calc/trunk/add-ins/polygons/
/calc/trunk/add-ins/solids/
/calc/trunk/add-ins/arithmetic/
/calc/trunk/add-ins/storage/

How can I maintain the add-ins folder when there is a new
release of "libcomplex"?

Each of the product vendor releases have several sub-folders,
but the project itself ("calc" in the above example) requires
that all of these sub-folders appear flat in the "libraries"
folder.

So the vendor branches look like this:

/vendor/libcomplex/current/arithmetic/
/vendor/libcomplex/current/arithmetic/addition.c
/vendor/libcomplex/current/storage/
/vendor/libcomplex/current/storage/database.c
/vendor/libcomplex/current/VERSION.txt

/vendor/libgeometry/current/polygons/
/vendor/libgeometry/current/polygons/triangle.c
/vendor/libgeometry/current/solids/
/vendor/libgeometry/current/solids/cube.c
/vendor/libgeometry/current/VERSION.txt

My initial reaction was to try something like the following:
        svn copy /vendor/libcomplex/current/* /calc/trunk/add-ins/
However, "svn copy" takes only one source and one destination
argument.

        svn delete /calc/trunk/add-ins/arithmetic/
        svn copy \
                /vendor/libcomplex/current/arithmetic \
                /calc/trunk/add-ins/arithmetic

But doing this (working-copy to working copy) results in the error
message "svn: Entry already exists".

I really would prefer the whole operation involve only a single
commit -- I would hate to have a "mixed bag" of versions sitting
around in the same folder accidentally!

Can someone please point me in the right direction, or help me with
search terms that might turn up relevant results?

Even just a definitive answer telling me that I'm "missing the point"
and "shouldn't be doing that" would be useful ;-)

Thanks,

-- 
Robert Hunter
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