What exactly do kevlar and helmets protect against?
A kevlar vest at $650 cuts incoming body damage by approximately half. Adding a helmet for a combined $1,000 also neutralises the bonus headshot multiplier from most guns. Key exceptions exist, though: the AK-47, SG553, and AWP all retain lethal headshot potential even through full armour. On T-side eco rounds, grabbing a helmet is almost always worth it.