If he expected to survive it he wouldn't have posted it all over facebook or called a dozen people.
He also would have negotiated with the police and turned himself in, they spent a whole lot of time trying to negotiate with him and it went no where. There was simply no way he was going to escape and avoid being blamed for the crime, he had heavily publicized the fact he did it!
The vest was likely to prevent getting taken out before he accomplished his mission.
You know, now that I think about it, he might've made changes in his plan when he realized he couldn't purchase the vest. Maybe his original intent was to survive?