Problem with tcsh

Maybe some other Unix expert here can help me. I'm trying to do something like this:

su - newuser -c "command"

What happens is that the command, which starts a fairly complex piece of software (vncserver), complains about not being able to find stuff it needs in the PATH. Having used "su -..." I would expect the PATH to be set.

Now, I tried exactly the same thing with another user account that used ksh instead of tcsh, and it worked fine.

I wound up having to pass two environment variables to set the PATH and HOME in order to get it working with tcsh.

Now, why would tcsh behave like that?