why do Federal agent usually wear Police vest?

Most federal agents wear business attire or plain clothes; they only wear bullet-proof vests in situations where armed offenders may be present.

The reason these vests are usually marked "police" is because that term is universally understood, whereas many federal agencies are quite obscure and not everybody knows about them. Also, immigrants who aren't familiar with US agencies might not understand the terms either. Pretty much everyone, however, knows what "police" means.

Because federal agents are police. They are federal law enforcement and police is a catch all phrase for any federal, state, or local police law enforcement agency. Also, in te most practical terms, its so they can protect themselves when getting shot at.

Stylin'...

EDIT: Feds Aren't Police. They are "peace officers". I have seen them argue over jurisdiction, it usually defaults to the feds.

And, ATF agents had "ATF" vests on a raid. I saw them, so did the whole neighborhood. Had their own "ATF" cars, too. No confusion.

Because they want to look smart and handsome too.