The unified sources VISTA is a DB VISTA rule, so it's much more flexible
than the normal rule. Also more expensive, though. I've used it at
several gigs with great success, but may try my luck with marketing
channels going forward.
The unified sources VISTA rule reports traffic for subdomains exactly
like you describe. It's a pain sometimes because you'll get some domains
like stumbleupon.com or craigslist.org that will show hundreds of
different referrers to your site. But for the case you describe it would
be very helpful. When I was at Carfax, for example, it was helpful to
see how much traffic we were getting from car listings on
dc.craigslist.org vs sf.craigslist.org.