I apologise for swarming an already overly populated topic, and I hope I didn't miss a forum post that already answers my question! 

Any creature hostile to you that moves to a space within 10 feet of the guardian for the first time on a turn must succeed on a Dexterity saving throw. 

My party encountered a vampire, the cleric laid out a beautiful guardian, and then the vampire used a legendary action to move. I believe legendary actions aren't turns and come in the blank space between turns, so I ruled that it didn't trigger the guardian. It then led me to wonder if a smart vampire would only use legendary actions to move, to avoid triggering the guardian attack. Thematically I thought it made sense, treating the guardian as a motion detector, and the vampire's legendary action was just too quick to trigger it. 

Was I wrong in this ruling? Do legendary actions count as a mini-turn that would trigger the guardians' attack? 

From the MM:

A legendary creature can take a certain number of special actions — called legendary actions — outside its turn. Only one legendary action option can be used at a time and only at the end of another creature’s turn.

A legendary action occurs on a turn: at the end of another creature's turn. They are not a mini turn, nor are they in the imaginary space between turns.

I'd say the vampire should have been affected by Guardian Of Faith.

A legendary action is just another type of action, and players or other effects can react to it unless stated otherwise in the description.

If it's useful, we have this in the Sage Advice Compendium:

Does the movement from a dragon’s Wing Attack legendary action draw attacks of opportunity if it moves out of reach?
Yes. Wing Attack does not protect the dragon from opportunity attacks.

 
EDIT: I got ninja'd by WolfOfTheBees :D