A landfall ability triggers whenever a land you control enters for any reason. It triggers whenever you play a land, as well as whenever a spell or ability puts a land onto the battlefield under your control.

A landfall ability doesn't trigger if a permanent already on the battlefield becomes a land.

Whenever a land you control enters, each landfall ability of the permanents you control will trigger. You can put them   on the stack in any order. The last ability you put on the stack will be the first one to resolve (As a result, you can have those abilities resolve in the order of your choosing.).

Felidar Retreat's second mode affects only creatures you control at the time the ability resolves, including creatures you control but that for some reason didn't get a +1/+1 counter. Creatures you begin to control later in the turn won't gain vigilance or get a +1/+1 counter.