(untested in 10.10.2) There is still no way to close the Preview Pane in the Mail app without using a physical mouse or trackpad. In older versions, one could at least use one of the sliders to do this with VoiceOver, but that was broken in Mavericks and remains so in Yosemite. Even the AppleScript command to close the Preview Pane is still not working, to my knowledge.
When not closed, the Preview Pane causes any message a user arrows past to be marked as "read", even if it was not actually read by the user. Since arrowing is the only way VoiceOver users can read emails, this poses a problem for those who rely on the read status. Thus, many VoiceOver users want to close the Preview Pane entirely.
Bug First Encountered
How often the bug occurs
Once you interact with the proper splitter. route the mouse to it with vo-cmd-f5. You must then physically move the mouse straight down until the splitter is closed, but there is no VoiceOver feedback as you move. VoiceOver commands should be able to move this splitter (by interacting and using vo-down) but that won't work.