For an armored vest I'd apply the default 0 concealability modifier, and since spotting something that is concealed is a success test with a threshold entirely up to the GM I'd play around with the difficulty. An obscured object is threshold 3, a hidden object is threshold 4, so I'd say depending on the amount of clothing covering it it's at least a threshold 3+ test. If the user was trying to actively conceal it, then the test becomes opposed instead.
Off-topic, but if synthetic cyberlimbs are "obviously artificial to the touch", wouldn't everyone with thermal vision mods instantly spot cyberlimbs since they (presumably) don't have the same temperature as the rest of the human body? Or is this one of those situations where you just handwave it for simplicity's sake.