I couldn't find an easy way to do this, so here you go. This just drops the z from the Vector 3, but you can optionally save it as a float.
// (c) copyright Hutong Games, LLC 2010-2012. All rights reserved.
using UnityEngine;
namespace HutongGames.PlayMaker.Actions
{
	[ActionCategory(ActionCategory.Vector2)]
	[Tooltip("Store a Vector3 XY component into a Vector2 XY component. Drops the Vector3 z component, you can optionally save it to a float.")]
	public class Vector3toVector2 : FsmStateAction
	{
		
		[UIHint(UIHint.Variable)]
		[Tooltip("The vector3")]
		public FsmVector3 vector3;
		[UIHint(UIHint.Variable)]
		[Tooltip("The vector2")]
		public FsmVector2 vector2;
		[UIHint(UIHint.Variable)]
		[Tooltip("Optional z value stored as float")]
		public FsmFloat zValue;
		public bool everyFrame;
		
		public override void Reset()
		{
			vector2 = null;
			vector3 = null;
			everyFrame = false;
		}
		
		public override void OnEnter()
		{
			ConvertVector3toVector2();
			
			if (!everyFrame)
			{
				Finish();
			}
		}
		
		public override void OnUpdate()
		{
			ConvertVector3toVector2();
		}
		void ConvertVector3toVector2()
		{
			vector2.Value = new Vector2(vector3.Value.x,vector3.Value.y);
			zValue.Value = vector3.Value.z;
		}
	}
}