Domain Mapping + Membership + Multisite

My site at ws.vncradio.com is a subsite on my multisite install at vncradio.com. I'm ready to deploy the site at virtualnewscenter.com, but when I do I have two issues:

1) login redirect loop, whether trying to access virtualnewscenter.com/wp-admin or ws.vncradio.com/wp-admin
2) membership logins do not cause an error or redirect, but the user still has only visitor access to the site.

in looking at wp-config.php, I see that W3TC added ws.vncradio.com to the cookie domain setting...I have trimmed that back to vncradio.com. I've also tried all three settings for how multisite handles admin logins, to no effect. I suppose that makes sense, though, as membership users have a subscriber access role, not an admin role.

This issue only happens when I put virtualnewscenter.com in the domain mapping settings on ws.vncradio.com.

So far, I have not been able to find an answer to the specific combination of issues...though I suspect it will be farily obvious, since ProSites is supposed to tie all these things together anyway. I just need the membership funcitonality on one subsite of the multisite install, at this point, which is the reason I didn't try ProSites yet.

You will not see the issue at virtualnewscenter.com right now -- I've re-pointed nginx to our old (Joomla) site until I can resolve this issue with the new one. If need be, I can point one of my other domains to this site so we can work through it.

Thank you in advance for any troubleshooting steps, resource links, or other help you can provide.