Unity シェーダー　頂点を引き伸ばす

前提・実現したいこと

描画しているメッシュの頂点をシェーダーで引き延ばしたい

該当のソースコード

using System.Collections;
using System.Collections.Generic;
using UnityEngine;
using UnityEngine.EventSystems;

public class PlacementDraw : MonoBehaviour
{
    public Material material;
    Mesh mesh;
    List<Vector2> ObjectPositionList = new List<Vector2>();
    public Placement.StateType stateType;
    // Start is called before the first frame update
    void Start()
    {
        mesh = new Mesh();
        mesh.vertices = new Vector3[] {
        new Vector3 (0, 0, 0),
        new Vector3 (0f, 1f, 0),
        new Vector3 (1f, 0f, 0),
        new Vector3 (1f, 1f, 0),
    };

        mesh.uv = new Vector2[] {
        new Vector2 (0f, 0f),
        new Vector2 (0f, 1f),
        new Vector2 (1f, 0f),
        new Vector2 (1f, 1f),
    };

        mesh.triangles = new int[] {
        0, 1, 2,
        1, 3, 2,
    };

        mesh.RecalculateNormals();
        mesh.RecalculateBounds();
    }

    // Update is called once per frame
    void Update()
    {
        PlacementObject();
        for (int i = 0; i < ObjectPositionList.Count; i++)
        {
            Graphics.DrawMesh(mesh, ObjectPositionList[i], Quaternion.identity, material, 0);
        }
    }
    void PlacementObject()
    {
        var pos = Camera.main.ScreenToWorldPoint(Input.mousePosition);
        var x = Mathf.FloorToInt(pos.x);
        var y = Mathf.FloorToInt(pos.y);
        var pos2 = new Vector2(x, y);
        Placement placement = GetComponent<Placement>();
        if (Input.GetMouseButton(0) && stateType == placement.state)
        {
            if (EventSystem.current.IsPointerOverGameObject())
            {
                return;
            }
            ObjectPositionList.Add(pos2);
        }
    }
}

Shader "Unlit/StencilObject"
{
    Properties
    {
        [PerRendererData] _MainTex ("Sprite Texture", 2D) = "white" {}
        _Color ("Tint", Color) = (1,1,1,1)
        [MaterialToggle] PixelSnap ("Pixel snap", Float) = 0
    }

    SubShader
    {
        Tags
        { 
            "Queue"="Transparent" 
            "IgnoreProjector"="True" 
            "RenderType"="Transparent" 
            "PreviewType"="Plane"
            "CanUseSpriteAtlas"="True"
        }

        /*Stencil
        {
            Ref 1
            Comp equal
        }*/

        Cull Off
        Lighting Off
        ZWrite Off
        Blend One OneMinusSrcAlpha

        Pass
        {
        CGPROGRAM
            #pragma vertex vert
            #pragma fragment frag
            #pragma multi_compile _ PIXELSNAP_ON
            #include "UnityCG.cginc"

            struct appdata_t
            {
                float4 vertex   : POSITION;
                float4 color    : COLOR;
                float2 texcoord : TEXCOORD0;
            };

            struct v2f
            {
                float4 vertex   : SV_POSITION;
                fixed4 color    : COLOR;
                float2 texcoord  : TEXCOORD0;
            };

            fixed4 _Color;

            v2f vert(appdata_t IN)
            {
                v2f OUT;
                OUT.vertex = UnityObjectToClipPos(IN.vertex);
                OUT.texcoord = IN.texcoord;
                OUT.color = IN.color * _Color;
                #ifdef PIXELSNAP_ON
                OUT.vertex = UnityPixelSnap (OUT.vertex);
                #endif

                return OUT;
            }

            sampler2D _MainTex;
            sampler2D _AlphaTex;
            float _AlphaSplitEnabled;

            fixed4 SampleSpriteTexture (float2 uv)
            {
                fixed4 color = tex2D (_MainTex, uv);

#if UNITY_TEXTURE_ALPHASPLIT_ALLOWED
                if (_AlphaSplitEnabled)
                    color.a = tex2D (_AlphaTex, uv).r;
#endif //UNITY_TEXTURE_ALPHASPLIT_ALLOWED

                return color;
            }

            fixed4 frag(v2f IN) : SV_Target
            {
                fixed4 c = SampleSpriteTexture (IN.texcoord) * IN.color;
                c.rgb *= c.a;
                return c;
            }
        ENDCG
        }
    }
}

追加スクリプト

using System.Collections;
using System.Collections.Generic;
using UnityEngine;

public class PlacementShadow : MonoBehaviour
{
    public Material material;
    Mesh mesh;
    List<Vector2> ObjectPositionList = new List<Vector2>();
    // Start is called before the first frame update
    void Start()
    {
        mesh = new Mesh();
        mesh.vertices = new Vector3[]
        {
            new Vector3(0f,0f,0f),
            new Vector3(0f,1f,0f),
            new Vector3(1f,0f,0f),

            new Vector3(0f,1f,1f),
            new Vector3(1f,0f,1f),
            new Vector3(1f,1f,1f),
        };
        mesh.uv = new Vector2[]
        {
            new Vector2(0f,0f),
            new Vector2(0f,1f),
            new Vector2(1f,0f),
            new Vector2(0f,1f),
            new Vector2(1f,0f),
            new Vector2(1f,1f),
        };
        mesh.triangles = new int[]
        {
            0,1,2,

            3,2,1,
            2,3,4,

            5,4,3,
        };
        mesh.RecalculateBounds();
        mesh.RecalculateNormals();
    }

    // Update is called once per frame
    void Update()
    {
        Placement();
        for(int i = 0;i < ObjectPositionList.Count; i++)
        {
            Graphics.DrawMesh(mesh,Vector3.zero, Quaternion.identity, material, 0);
        }
    }
    void Placement()
    {
        var pos = Camera.main.ScreenToWorldPoint(Input.mousePosition);
        var x = Mathf.FloorToInt(pos.x);
        var y = Mathf.FloorToInt(pos.y);
        var pos2 = new Vector2(x, y);
        if (Input.GetMouseButton(0))
        {
            ObjectPositionList.Add(pos2);
        }
    }
}

試したこと

正方形に描画しているメッシュの３つの頂点を引き伸ばして、ボリュームシャドウのようにしたかったのですが、どうすればシェーダーで描画しているメッシュの頂点をいじれるのか、調べても分かりませんでした。
どのようにすればシェーダーでverticesを変更することができますか？
回答お願いします。