No, it was Vicksburgh.
Both victories for the north came in the first week of July, only days apart. But let's consider the following:
Grant withdraws from Vicksburg -- Pemberton then had about 30,000 men, in weakened condition and low on ammo. Grant had 75,000 plus troops, all relatively fresh (compared to Pemperton's men). Grant would have withdrawn, to rest and replenish his men, but likely Pemberton, or along with others in the area, would not have been able to counter attack.
Lee Prevails at Gettysburg -- Meade would have had to retreat, east towards DC, leaving the way north open for Lee. Both armies were relatively close in size, with Meade having the edge. Lee would have been only 40 miles from Harrisburg, a large city untouched by the war and a city full of much needed supplies. Lee's troops would have foraged the countryside and likely held up at Harrisburg so as to not overextend their southerly supply lines. The panic an invasion of PA would have sent shockwaves through the north.
You are correct that Vicksburg was a devastating loss for the south. But I think a Lee victory at Gettysburg would have had a larger immediate impact on the war.
Truth be told though, the numbers just did not add up for the south, who quickly were running out of men, and supplies.