It isn't possible to pass null as null into a .NET method that has a parameter of type String by Kirk Munro

You can work around the problem by using .Net reflection. For example, instead of:
    $obj.DoSomething($a,$b,$null)
...you can do:
    $params = @($a,$b,$null)
    $obj.GetType().GetMethod("DoSomething").Invoke($obj, $params)

One way you can workaround the problem is to write a cmdlet to wrap the .Net call you need to make. This is a signifant overhead for what i want to do.

I have included the code to fix my problem below.

using System;
using System.Collections.Generic;
using System.Text;
using System.Management.Automation;
using System.Collections;
using Microsoft.TeamFoundation.VersionControl.Client;

namespace WindowsPowerShell1
{
    [Cmdlet(VerbsCommon.Get, "Label", SupportsShouldProcess = true)]
    public class GetLabel : Cmdlet
    {
        private string label;
        private string scope;
        private VersionControlServer vcs;
        private string user;
        private bool includeItems;

        #region Parameters
        [Parameter(Position = 0,
            Mandatory = true,
            ValueFromPipelineByPropertyName = true,
            HelpMessage="The label name - * is wildcard")]
        [ValidateNotNullOrEmpty]
        public string Label
        {
            get { return label; }
            set { label = value; }
        }
        [Parameter(Position = 1,
         Mandatory = true,
         ValueFromPipelineByPropertyName = true,
         HelpMessage = "Scope")]
        [ValidateNotNullOrEmpty]
        public string Scope
        {
            get { return scope; }
            set { scope = value; }
        }
        [Parameter(Position = 2,
         Mandatory = true,
         ValueFromPipelineByPropertyName = true,
         HelpMessage = "VersionControlServer")]
        [ValidateNotNull]
        public VersionControlServer Vcs
        {
            get { return vcs; }
            set { vcs = value; }
        }
        [Parameter(Position = 3,
             Mandatory = false,
             ValueFromPipelineByPropertyName = true,
             HelpMessage = "VersionControlServer")]
        public string User
        {
            get { return user; }
            set { user = value; }
        }
        [Parameter(Position = 4,
             Mandatory = true,
             ValueFromPipelineByPropertyName = true,
             HelpMessage = "IncludeItems")]
        public bool IncludeItems
        {
            get { return includeItems; }
            set { includeItems = value; }
        }
        #endregion

        protected override void ProcessRecord()
        {
            try
            {
                WriteObject(Vcs.QueryLabels(label, Scope, User,
IncludeItems));
            }
            catch (Exception)
            {
            }
        }
    }
}