first you can replace "Highborne Soul Mirror" with 13 or 14 depending if it's on the "top" or "botom" trinket slot.
Then you can indeed remove the "vanish" in the "#showtooltip", but if you don't want to then just keep it. Anyway that's what I got with some refactoring without changing at all your macro's behaviour :
Code:
#showtooltip Vanish
/stopattack [nomod][mod:ctrl]
/use [nomod]Highborne Soul Mirror
/use [mod:shift]Cloak of Shadows;[mod:alt]Fan of Knives;[mod:ctrl]Shadowmeld(Racial);Vanish
/use [mod:ctrl,nocombat]!Stealth;[mod:ctrl]Cheap Shot
which is 229 char
if you always keep your mirror in the same trinket slot (13 or 14) and don't mind about vanish not always being on top of your macro then you can use :
Code:
#showtooltip Vanish
/stopattack [nomod][mod:ctrl]
/use [nomod]13
/use [mod:shift]Cloak of Shadows;[mod:alt]Fan of Knives;[mod:ctrl]Shadowmeld(Racial);Vanish
/use [mod:ctrl,nocombat]!Stealth;[mod:ctrl]Cheap Shot
which is 203 210 chars.
edit : beaten by DesolatedMaggot, who did the same thing as me but in an other order, which won't change anything as long as you never use "ctrl-shift" or "alt-shift" together...
edit 2 : actually you shoud keep #showtooltip vanish if you want vanish instead of your trinket showing