Audio problem: When Vince turns up at the BBQ he sees Brian and walks off - Dom shouts "Vince, give us a hand" and he replies "looks like you got all the help you need, brother" - his voice doesn't match his mouth.
Continuity mistake: When the first street race takes place with the four cars, it is evident that the race takes place along a straight stretch of road. However, when the race finishes, everyone who was at the start line just a minute ago has magically appeared at the finish line, miles down the road.
Answer: There are two ideas of why the police did not come around. Firstly they might have found out after the car chase that Brian was an undercover cop so it would be more effective to allow him to try and discover information instead of going around and arresting Dom. The other idea is that the police might not have enough to arrest him. Dom was not the one driving when they found him and other than racing, it might be tough to arrest him.
Lummie ★
Cops are usually completely unaware of an undercover cop's identity. The more, realistic reason is that the cop didn't ID Dom and since Brian's car was later destroyed, there's nothing for them to investigate. Afterwards, they might ID Dom, but there's an investigation on him, so they'd leave him alone.