Unable to purchase Kevlar in CS:GO

Kevlar + Helmet:

• Reduces damage by a fixed percentage depending on the gun.
• Only applies when hit in the chest, stomach and arms for the kevlar.
• Only applies when hit in the head for the helmet.
• Legs do not receive the damage reduction from kevlar.

The way it works (explained using an example):

• The MP9 has a fixed damage reduction percentage of 50%.
• If no armour, at close range and hit in the stomach, the MP9 inflicts 32 points of health damage.
• With armour, if hit at the same range and place, the MP9 inflicts 16 points of health damage and 16 points of armour damage.
• With only 4 points of armour left, if hit at the same range and place, the MP9 inflicts 28 points of health damage and 4 points of armour damage. The full damage reduction is applied, but whatever can't be removed from points of armour is removed from health instead.
• Damage to both armour and HP is rounded before application. Health damage is rounded down and armour damage is rounded up.

Fixed damage reduction percentages for each gun can be found here.

My example gun damage was obtained from this spreadsheet.
An alternative I found is here. The numbers are slightly different from one another, however I do not know which is the more recent and I am unable to check myself at the moment.

Finally, I found a nice topic on the steam forums. It helped me with a few things I was unsure of. Take a read here.

Hope that helps.

Armor, as long as you have it, will have the same reduction regardless of how much. Example: 50 armor reduces bullet damage just as much as 100. The only time you need to rebuy is if your armor will drop to 0 during the round (2-3 bullets worth) since 0 armor provides no bullet damage reduction. Rule of thumb typically is around 40, you should rebuy.

Sources:

• http://counterstrike.wikia.com/wiki/Kevlar_Armor/Helmet
• http://www.hattongames.com/2013/03/quick-tips-should-i-always-wear-protection/
• http://www.reddit.com/r/GlobalOffensive/comments/1dzj8h/when_should_i_rebuy_a_kevlarhelmet/