How to correctly use synclock to only allow one call to a function at a time

Question

I have code that sends a ping to a batch of computers, and opens a VNC session to any that respond that they are online.

The pings are sent in a parallel.foreach loop, and an event is raised when a reply is receieved.

Despite putting a synclock block around the AddHost code, it is still being entered by multiple threads simultaneously.

I think it has something to do with the code being called by a raiseevent on another object.

This is the method that is handling the event

Private Sub AddHost(hostname As String)
    ' panesList is an list(of RemoteDesktop) prepopulated with  36 individual RemoteDesktop objects
    Dim vnc As RemoteDesktop = panesList(Position)
    vnc.GetPassword = New AuthenticateDelegate(Function() vncPassword)
    SyncLock Semaphore
        Try
            If Position = 36 Then
                Return
            End If

            vnc.Connect(hostname, 0, True, True)
            Position += 1

        Catch ex As Exception

        End Try
    End SyncLock

End Sub


Private Sub HandleConnect(hostFilter As String)
    AddHandler OnlineFinder.LiveComputer, AddressOf AddHost
    Dim bg As New Task(Sub() OnlineFinder.CheckFilteredASync(hostFilter))
    bg.Start()
    ' AddHandler OnlineFinder.SearchComplete, AddressOf Finished
End Sub

From the object OnlineFinder.vb

    Public Sub CheckFilteredASync(filterString As String)
    Dim Ctable As DataTable = CTableAdapter.GetDataByPartName($"{filterString}%")
    Dim clist As New List(Of String)
    For Each drow As DataRow In Ctable.Rows
        clist.Add(drow.Field(Of String)("Name"))
    Next
    Searching = True
    Dim parallelLoopResult = Parallel.ForEach(clist, Sub(site) Ping(site))
    RaiseEvent SearchComplete()
End Sub

    Private Sub Ping(host As String)
    If ValidPing(host) Then
        RaiseEvent LiveComputer(host)
    End If
End Sub

How should I be doing this so only one event is processed at a time (but it doesn't hang the UI)