Vinci, who stunned Venus's younger sister Serena in the semi-finals of the U.S. Open last month, had fought back from 3-0 down to take a topsy-turvy first set featuring five breaks of serve.
But she failed to hit the heights thereafter as Venus, who won her 700th match on tour earlier this week, took command in the second set, racing ahead 5-1 before forcing the decider.
An early break put the American up 3-1 in the third set but she stumbled when serving for the match at 5-3, with the Italian, chasing a best-ever world ranking of 11th this week, breaking and then holding her own serve to make it 5-5.
It was then Vinci's turn to wobble when serving for the match. The doubles specialist, who lost the U.S. Open final to compatriot Flavia Pennetta, failed to convert a match point and was broken as Williams forced a tiebreak.
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Williams secured an early mini-break as she wrapped up the match in just under three hours to reach her 77th career final and first at the fledgling tournament staged in the home town of China's first grand slam singles champion Li Na.
"Definitely watching the match at the U.S. Open, I learned a lot from Serena," Williams told reporters after her win.
"If I could, I'd give my win to Serena at the U.S. Open -- unfortunately it doesn't work like that."
Muguruza suffered an ankle injury but kept up her bid to qualify for the eight-woman end-of-year WTA Tour finals in Singapore by coming through a hard-fought clash against sixth seed Kerber.
The Venezuelan-born 21-year-old called the trainer at 3-3 in the second set tiebreaker but recovered to claim the win.
(Writing by Patrick Johnston in Singapore; Editing by Amlan Chakraborty and Clare Fallon)
