On top of that, Tribunals can still be assassinated whilst on duty with vigilance up

This is solely dependant on how often you type where. Ofcourse you should know that though, that the assassin has to walk to key rooms to be able to stalk you, and no one uses those rooms within the city to log in/out. So its easy to see them heading into said rooms, etc.

I agree with Xaanix, tribunal is an easy option if you worried about Empire, or getting assassinated.

There are two scion powers that can allow you to prevent thiefs and assassins bothering you, even if you can't see them

Um...I wouldn't use the word "preventing" because its not quite accuracte. A smart assassin can get around them rather easily actually. Vigilance is a LOT harder to get around than these powers. Not to mention Scion has THREE ENEMIES, two of which often raid for the scepter. How often do the Tribs have their Item? Almost all of the time, Outlander is too piss-weak and without enough muscle to take it.