My Cousin Vinny

Driving through Alabama, Billy Gambini and Stan Rothenstein, students from New York, shop at a convenience store and accidentally shoplift a can of tuna. After they leave, the store clerk is shot and killed, and Billy and Stan are arrested in connection with the murder. Due to circumstantial evidence and a confession to shoplifting misconstrued as one to the shooting, Billy is charged with murder, and Stan as an accessory. Billy's mother tells her son that there is an attorney in the family: his cousin Vinny. Vincent LaGuardia Gambini travels there, accompanied by his fiancÃ©e, Mona Lisa Vito. Although he is willing to take the case, Vinny is a personal injury lawyer from Brooklyn, New York, newly admitted to the bar on his sixth attempt, and with no trial experience.
Vinny manages to fool the trial judge, Chamberlain Haller, about being experienced enough for the case. His ignorance of basic court procedures and dress code, and abrasive and disrespectful attitude, cause the judge to repeatedly hold him in contempt. Much to his clients' consternation, Vinny does not cross-examine any of the witnesses in the probable cause hearing. Except for the murder weapon, it appears that the district attorney, Jim Trotter III (Lane Smith), has an airtight case that will lead to convictions. After Vinny's poor showing at the hearing, Stan fires him and uses the public defender, John Gibbons (Austin Pendleton), and nearly convinces Billy to do the same.
Despite some further missteps in the trial, Vinny shows that he can make up for his ignorance and inexperience with an aggressive and perceptive questioning style. The public defender is unprepared, passive, and has a debilitating stammer, but Vinny discredits the testimony of the first witness. Billy's faith is restored, and Stan fires the public defender.
Vinny's cross-examinations of the remaining two eyewitnesses are similarly effective, but on the trial's third day, Trotter produces a surprise witness, FBI analyst George Wilbur, who testifies that the pattern and chemical analysis of the tire marks left at the crime scene are identical to the tires on Billy's Buick Skylark. With only the lunch recess to prepare his cross-examination and unable to come up with a strong line of questioning, Vinny realizes that one of Lisa's photos holds the key to the case: the flat and even tire marks going over the curb reveal that Billy's car could not have been used for the getaway. They could only have been made by a car with an independent rear suspension and positraction; Billy's Skylark does not possess these features, but the similar-looking Pontiac Tempest would.
After requesting a records search from the local sheriff, Vinny drags Lisa into court to testify as an expert witness. (Both Vinny and Lisa had worked as mechanics in her father's garage.) During Vinny's questioning, Lisa comes to the same conclusion and testifies that the only vehicle that could plausibly make the escape and be mistaken for Billy's 1964 Skylark is a 1963 Pontiac Tempest with the same color and tires. Vinny re-calls George Wilbur, who confirms this. Vinny then re-calls the local sheriff, who testifies that two men who fit Billy and Stan's descriptions were arrested in another county for driving a stolen Pontiac Tempest, and were in possession of a gun of the same caliber used to kill the clerk. Trotter moves to dismiss all charges, and the film ends with kudos trading and Vinny and Lisa bantering about their future wedding plans.

Question: Who is proving to be a good lawyer?
Answer: Vinny.