Vendor software update and branches

[Mark]

I've been reading the manual and doing some testing but can't come up
with a procedure for the following.

We have 3rd party software and a vendor has givin us a new version.  We
will be working on integrating this new version into our code base, but
at the same time can not change the main trunk until we are ready.

The inital vendor software was imported on a vendor branch.

I have set up a test where I cvs import a directory into a test
repository.  Then I create a workspace, called sandbox 1, check out the
code, and commit a few modifications.

Then I change the 'vendor source' and cvs import again using a new
release tag.  At that point, cvs update in sandbox 1 wants to extract
the new versions (except where I made changes), but I need to keep the
main trunk (HEAD) in tact with the vendor code it has always been
using.

The only way I can see to do this is
-Create a new branch, called vendor_update
-Check out vendor_update
-copy in the new vendor drop, manually removing elements accordingly
-Check into vendor_update

The problem with this is we are not able to easily diff the vendor
releases.

What is the best way to get the proper outcome?

Regards,
Mark