The reason why Vincent Wei is written as the ML is because he has more screen time in comparison with Dai Gao Zheng. I think it's this way because:
Maid's Revenge: Dai Gao Zheng is ML, Chen Fang Tong is FL, Vincent Wei is villain → story's main focus is on romance and less on revenge → Dai Gao Zheng & Chen Fang Tong have more screen time in comparison with Vincent Wei
Bride's Revenge: Vincent Wei is ML and villain, Qu Meng Ru is FL and Dai Gao Zheng is SML & FL's love interest → story's main focus is on revenge and slightly less on romance → Vincent Wei & Chen Qu Meng Ru have more screen time in comparison with Dai Gao Zheng (his screen time is less due to his absence for a portion of the show where he undergoes character development to take a major role in getting revenge with Qu Meng Ru against Vincent Wei)
I'm not arguing that there is no romance in Bride's Revenge, however, I believe in comparison with Maid's Revenge, Bride's Revenge focuses more on Qu Meng Ru and Dai Gao Zheng working together to get their revenge while also falling for each other in the process. So technically since the story focuses more on the revenge aspect than romance, That's why Vincent Wei is the ML in this drama since he is the main villain and not the love interest. So, even though Dai Gao Zheng has less screen time, his absence is somewhat better for the story. His disappearance allows Qu Meng Ru's character to become more confident and daring, as well as allowing her to take major actions to find answers and move towards her revenge. This gap time is also really good for Dai Gao Zheng's character, who didn't die as Mu An He, and returns as Tang Zi Feng while hiding his real identity as Mu An He. This gap period allows his character to break out from his somewhat naive and obedient shell and become stronger both mentally and physically to take on Vincent Wei and take back everything that originally belongs to him.
To summarise, the reason Vincent Wei is the ML and Dai Gao Zheng is the SML is because the story primarily focuses on revenge, not romance, and being the main villain, Vincent Wei ends up having more screen time in comparison with Dai Gao Zheng, who while also being a major character of the show, has less screen time than Vincent Wei due to the building up and evolvement of his character.
