One of the reasons for buying a wetsuit at your LDS rather than online, fit matters for comfort and ease of use and you can get it right first time.
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If you were wearing a body suit and a Mysterioso M-tech shirt, what would go closest to your skin, the body suit or the Mysterioso M-tech shirt? I am guessing that it would be the Mysterioso M-tech shirt but my guesses have been wrong before.g1138:underneath it all I wore a Mysterioso M-tech Fleech T-shirt.
(A 5mm vest sounds quite thick though, never seen anyone using those.)
Cricket at Otter Bay suggested that the vest go on the outside. That way, the same wetsuit can be worn with and without a vest. Hard to argue with the logic. The way she makes the skins for the neck of the vest and the neck of the hood there is no water infiltration, so an outer vest makes more sense all around.
She also suggested a separate hood and vest. That way, I could take off the hood, which she said most people want to take off right away, without having to wriggle out of the vest immediately. This is useful also for being suited up before you go into the water. Sometimes you have to wait before you go into the water, and I have seen comments here on SB that divers sometimes get hot with the 12 mil hood on.