IMO, you shouldn't be using mouseovers for PVE anyway, just focus your secondary target and use "cast on focus" macros.
This is a great target/focus switch macro for V&T that will keep the grounded dragon as your target and the air one as your focus on phase changes:
Code:
/focus [target=focus, noexists]
/target [combat, noexists] focus
/clearfocus [nocombat, noexists]
/stopmacro [target=focus, noexists]
/target focus
/targetlasttarget
/focus [exists]
/targetlasttarget
Edit: Use Tidyplates to keep tack of your DoTs on more than your target and focus, like on Maloriak and click to target while VT is casting.