Choking Vines will make any creatures it targets become blocked, even if the creature had blocking restrictions on it. For example, you can make a creature with Islandwalk become blocked even if the defending player has Islands. This is because evasion abilities only prevent creatures from being assigned to block them.
You can declare zero targets, with X=0.
Choking Vines isn't particularly effective against creatures with trample. Since the creature with trample has no blocker to assign damage to, all the damage will be assigned to the defending player.
You may target a blocked creature with Choking Vines. Choking Vines will deal 1 damage to it, and have no other effect on it.
If a previously unblocked creature becomes blocked as a result of Choking Vines, any "When this creature becomes blocked" abilities that creature has will trigger. Any "When this creature becomes blocked by a creature" abilities that creature has won't trigger.