We got our first repeat customer last week and they couldn't log into their account. I finally tracked down the issue. Maybe I misunderstood how this feature works in the 'Registration' tab. When I check the 'Log out shopper before and after purchase (from Joomla)' box, returning customers are unable to login. Maybe it's only meant for sites that only use guest checkout. Anyway, in my testing it looks like VirtueMart is logging you in but nothing happens, you're right back where you started. There are no error messages either.
I thought I had a corrupt installation so I uninstalled and reinstalled OnePage Checkout. Logging in during checkout worked great until I customized my settings. So I went through them one by one and this is the only one that stops repeat customers from logging in. Let me know if I shouldn't have been using it -- we want customers to be able to checkout whether they are registered or not.