Sizing.
The trick to body armor, concealable or otherwise (I've worn Point Blank IBA and SAPI plates through two Iraq tours now) is sizing.
Your regular shirt size IS NOT THE SAME! Would be great if it were so, but there a too many places a vest must fit and fit CORRECTLY if it's going to work.
Buy in person if you can. If not, at least try and get a chance to try some on somewhere. Find out your sizing requirements before you buy, especially if you buy online.
Also, your style of dress may make a difference. A vest that will conceal under a shirt and tie will also conceal under a sweatshirt, but likely not vice versa.
The key points are the chest, abdomen, and side closures. If you get hit wearing one with too much room in the chest, you risk broken ribs (you'll probably have a few even with a properly fitted vest) and impact trauma to the heart/lungs. If it doesn't fit in the abdomen, you risk impact damage and internal bleeding in the guts. No one wants a ruptured liver. The side closures are the biggie. If you have too much excess, they'll bunch up under shirt and not conceal. Too short and you A) leave your kidney area exposed, and B) have a vest that is likely too tight or won't stay closed, again causing comfort/concealment issues.
Just some thoughts. Might be different for you, but things to consider nonetheless. Hope that's helpful.