Hey Jeff,
Are you using the current release of Vulcan from
http://code.google.com/p/vulcan/downloads/list?
I'm not aware of any issues with subversion authentication (though I haven't needed it on any of my projects since about a year ago).
Here's some generic troubleshooting ideas. There may be a problem in Vulcan but I don't have enough info to verify it.
Is your apache/svn server running on Windows? I've seen problems with NTLM authentication on windows machines interfering with Basic authentication.
Are you able to view the repository by logging in through a web browser using the same credentials you configured Vulcan to use?
Does anything interesting show up in the apache error log?
Good luck,
Chris