It was a cool scene....to make Vixen look cool. To show how dangerous she is.
And yes, Superman, Wonder Woman or Green Lantern could have hit him MUCH harder than Vixen.
Also Amazo has the powers of the entire Justice League...including Superman/Wonder Woman/Martian Manhunter's durability...so it shouldn't have done sh**.
HOWEVER, if you may recall, Red Arrow hit the Totem Necklace BEFORE Vixen divebombed him. Since the Totem Necklace was the source of Amazo's ability to process so many abilities, the arrow may have temporarily disrupt Amazo's powers so that his invulnerability turned off momentarily.