Heavily spoofed in History of the World Part 1
I was surprised to see that this fact wasn't mentioned on the movie connections page on this site. I've since added it.
Consider:
DeLuise's Nero was much in tribute to Ustinov's Nero - forcing people to listen to his awful poetry, lamenting his boredom, expressing feelings of martyrdom at being so oppressed by the needs of his people, etc. He is also depicted as rather stupid.
The Empress is depicted in both films as a calculating, lustful being.
Marcus Vinicius (Quo Vadis) is parodied by name as Marcus Vindictus. (History of the World)
In History of the World, Vindictus's approach to be presented to Nero is marked by the emperor's dresser reminding him repeatedly to "remember thou art mortal," whereas in Quo Vadis, Vinicius is being reminded repeatedly to "remember thou art but a man."