Kevlar can be essiantial on the later stages and on higher difficulties. Its price (300$) is really decent and shouldnt break the bank, unless you join a public game and have to start with the usual 200$.
Kevlar can be a real lifesaver and should always be bought if possible. Some perks even allow you to spawn with it if your perk is high enough.
Kevlar does not use up any of your "blocks" so it is reccommended to keep it in good condition, further supported by the fact you only need to pay for however much armor you need, not a whole new vest.