Seems to be an economical decision as well to go 5mm; haven't heard of hooded vests before.
Several of us have spoken about hooded vests on this thread. They are incredibly helpful to layer with a wetsuit to add additional warm.
This is the one I use and absolutely love. I can layer it under my 3mm or my 5mm as the conditions warrant.

Chillproof Vest with Hood - Mens
More than 25% of body heat is lost through the head and extremities of the body. The Sharkskin Chillproof sleeveless top with hood is designed to be worn underneath your wetsuit to provide the best warmth and insulation for colder dives. The hood protects against chafing from mask straps, keeps...
