ViewSVN does not seem to work for me but then it could just be that I need to figure out the complete url as http://servername/viewvc.cgi/ does not work. When I follow a link I get some script errors.
ViewVC take http://servername and your repo name and mangles them together to form something like:
http://servername/viewvc.cgi/?view=rev&root=repo&revision=21029
Now the viewvc.cgi section is hardcoded right now to viewvc and I have to change to to viewvc.cgi for the server I have access to, but after that change it works perfectly. We are trying to get websvn installed which I have been told has a nicer interface but who knows when that will happen.
The simplest fix is to require the viewvc portion added to the server name or the slightly more advanced version clutters up the advanced section with another box. If this is already in wide use then obviously suddenly needing to add viewvc everywhere will break most deploys.
as a workaround, can you select ViewSVN instead of ViewVC and enter the URL with viewvc.cgi there? does that work? (I'm not actually sure what viewvc plugin offers.. I've always selected ViewSVN and this seems to work when pointing to ViewVC)