In Europe most attacks with terroristic background were performed with bombs. The vest would not help very much when you are in the blast radius.
Those vests are easily spotted when you know what you´re looking for. In a magazine about using guns in selfdefence the author stated some interesting theories. When noone is carrying a weapon openly, the person with the vest would pose the most serious potential thread. This would turn this person into a primary target. It´s not too different for robberies. Body armor promises more loot and requires serious force. In conclusion they wrote that you should evaluate were you´re going, keep a low profile and be aware of the situation.
Concealed body armor could make LEOs nervous around here. That would be even worse if you don´t speak the local language.
BTW: Your chance of getting killed in a traffic accident is by far greater than getting harmed in a terrorist attack. At the risk of sounding cynic: The number of victims in the london bombings are lower than a week´s death toll in german traffic.