Having a vest with or without an armor plate makes all the difference.
A vest without armor can have only a small armor penalty like 10%, and something like 3-5 threshold, meaning 9-15 bullet damage stopped. Good for stealth.
A vest with armor will have much higher armor penalty, like 60%, But vastly better mechanical threshold, for example 16-20. Meaning you cant really do stealth, but will ignore 48-60 bullet damage, sentry bots will not even tickle you.