I would not say the 4th Maine bought any time for Vincent's brigade since the Confederate attackers (4-5 TX, 4-47-15 AL) were screened from their view by intervening woods. Although it is true the 44th Alabama was diverted away from Little Round Top to attack the Den.
However, a great argument could be made for the 2nd U.S. Sharpshooters. They essentially helped split the Confederate force into two roughly equal parts, which were then defeated in detail, in succession. The 4-5 TX and 4 AL attacked first and were effectively repulsed before the other half consisting of 47-15 AL made it down Round Top and entered the fight. It was the Sharpshooters who accomplished this diversion, and also lured away Company A of the 15th Alabama (about 50 men), a sizable force that would have been added to Col. Oates' right flank, perhaps enough to tip the fight in his favor.
Let's not forget the handful of signalmen on Little Round Top who waved their flags on the summit and compelled Longstreet's lengthy countermarch, and also the lost water detail from the 15th Alabama that required Oates to rest his men and himself for a few minutes after climbing up Big Round Top.