Stone warden and Stone vines

[Stenag]

I have some questions about the Stone vines and line of sight.
First, do I need to see my enemies to try to catch them ? And if I break LoS after catching them, do the vines break ?

Also, which stats are used to decide if the vines breaks or not ?

[SmallTownGuy]

I think LoS is required. But I've had vines kill rogues without apparently breaking stealth, so seeing the target may not be required.