I just finished getting the chest, diamond and belly pieces attached. I hot glued the magnets to the vest as well. Once you find your ideal positioning I find it much better to do this so you can take the pieces on and off at will and not have free magnets jumping around and sticking to each other. It also speeds up the application of the pieces at a later date.
RKD I am also stumped as to what you are doing with the neckpiece. Once you have the 2 bolts attached, then what? Could you expand on that part of your OP please? I am guessing you cut holes in the vest for the bolts to go through and then you attach via nuts on the inside? I don't want to do that until I get some confirmation though.
Another tip I have is to not skimp on the strength of the magnets if using an Arkady vest. It's double thickness and the 7lb magnets I got are just enough to hold things in place, and moving around/ flexing through the chest plate region too much will cause them to come detached. You could probably use the 10lb magnets with this thick vest. Definitely use 10lb magnets for the shoulder bells.
I also recommend using magnets on the armour *and* the vest, vs just a piece of metal on the vest. You will want the strength of attachment that a couple 7-10lb magnets will give you over a single stronger magnet (that will have more of a chance to interfere with the others near by) and a piece of metal.