Option Strict-compliant, safer, and more customizable version:
Code:
    Private Sub MakeRound(ByVal c As Control, Optional ByVal radius As Integer = 20)
        Dim p As New Drawing2D.GraphicsPath()
        p.AddArc(New Rectangle(0, 0, radius, radius), 180, 90)
        p.AddLine(20, 0, vObject.Width - 20, 0)
        p.AddArc(New Rectangle(vObject.Width - radius, 0, radius, radius), -90, 90)
        p.AddLine(vObject.Width, 20, vObject.Width, vObject.Height - radius)
        p.AddArc(New Rectangle(vObject.Width - radius, vObject.Height - radius, radius, radius), 0, 90)
        p.AddLine(vObject.Width - 20, vObject.Height, 20, vObject.Height)
        p.AddArc(New Rectangle(0, vObject.Height - 20, 20, 20), 90, 90)
        p.CloseFigure()

        c.Region = New Region(p)
    End Sub
P.S. radius should actually be diameter.