If you ask Siri to search your inbox and you get results, you can review them in the 'conversation' scroll area > 'mail' card. However, if you want to open one in the Mail app, normal VO commands will not work to do so.
Bug First Encountered:
How often the bug occurs:
Route the mouse to the message you want to open with vo-cmd-f5, then double click (with the mouse, or vo-shift-space).