Not quite sure yet. Once I have a clear idea on how I am doing my under suit then I will know. Velcro is cheap and easy but straps will prevent no accidental armour slips so most probably straps.
In case anyone wondered how I attached the parts of the torso together here it is. Until this morning I was using velcro but that was too fiddly and needed a much easier way to attach them together. So I ordered 4 X 40mm side release buckles from Ebay and glued them directly onto the inside of the armour. This means now the 2 parts can clip together with ease. I can almost do all 4 on my own but 1 of the parts is slightly twisted and my brain can't cope with trying to work something out when I can't see the parts, they are upside-down and slightly twisted.