Absolutely. Body armor is designed to stop projectiles, not bladed weapons. There are different grades of body armor, as well. Better grades are heavier and restrict movement more, but all could be penetrated by a blade if the assailant were so inclined.
Besides, body armor can be easily avoided with a bladed weapon with pinpoint accuracy. It's much harder to be accurate with a firearm.
Body armor reduces the pressure of a projectile using Kevlar, which distributes the force over a larger surface area for a brief moment in time.
Pressure = Force / Surface Area of the point of impact.
Force = Mass x Acceleration.
A knife or sharp instrument has a pinpoint area of impact coupled with a much greater mass than a bullet, especially when wielded by a human being whose mass adds to its mass.
A bullet does have more acceleration, though, but it's not enough to cancel out the other factors. Also, a bullet only applies force for an instant, whereas a knife wielded by a person can continually be pressed into the vest for a longer period of time, which makes it much harder to stop.
Body armor is not specifically designed to stop bladed instruments. As the above poster mentioned, some higher grade body armor has a steel plate, which makes it very heavy, uncomfortable, and restrictive, so many people do not wear the steel plates for those reasons.