How far can a bulletproof vest protects?

There are many different levels of protection. What cops wear won't stop more than .357 Magnum at best. What our soldiers wear will stop several 7.62x39mm rounds but after a few hits the rifle protection plate shatters to pieces and they loose protection from rifle rounds, it is still good for pistol and shrapnel though. Dragon Skin has been shown to protect covered areas from a grenade blast at 0 feet but the Army says it doesn't last long enough for use (it wears out from simply existing, according to them... many people claim it to be a money issue though.)

It varies based on what armor is being used.

As others have said in many places they are illegal or there are restrictions when purchasing them. But the real deal is why would you need one? Say something happens on the street or someone breaks into your home. It would look like you are inviting trouble, impersonating an official. If at home. You have the wherewithall to put on a vest if you wake in the night? You will be charged or at least lose in a civil suit even if cleared criminally. There really should be no reason for a civilain to have a vest. In some places you can have one. But that $350 will cost you later if you have it. Sure could be fun to just own but again why? Think Columbine, VA Tech and such. If you have a weapon and are well practiced a normal citizen does not need a vest. Even if you buy one $350 it will buy you a piece of junk.

A bullet proof vest will not protect you from a pointed rifle round even at 500 yards. They are meant for round nose pistol rounds at close range and even they will leave one hell of a bruise on you and maybe even a cracked rib depending on the power of the round. A mag. round like a .357, .44 mag will leave a huge bruise. I've shot through 1/4 inch plate steel at 100 yards with my M-16A1. It left a nice little hole through it.

The heaviest vest out there will provide the protection below:
Type IV
(Armor Piercing Rifle) This armor protects against .30 caliber armor piercing (AP) bullets (U.S. Military designation M2 AP), with nominal masses of 10.8 g (166 gr) at a reference velocity of 878 m/s (2880 ft/s ± 30 ft/s). It also provides at least single hit protection against the threats mentioned in [Types I, IIA, II, IIIA, and III].

There are different levels of protection. Some have a ceramic plate, while others have a steel plate. As for their protective differences, couldn't tell you. From experience, I would prefer the ceramic. When hit, it is compromised like the steel plate, but it spreads the impact over the entire plate. So you get a nasty welt in the shape of the ceramic plate, rather than a broken rib. Ulitimately, it doesn't matter what they are hit by. Once hit, the plate is discarded.