The KordaMentha Real Estate-Australia rider broke away during the stage and survived to beat world time trial champion van Vleuten (Mitchelton-Scott) and Chloe Hosking (Ale Cipollini), who won the bunch kick for third.
“It is so good, victory tastes so sweet,” Chapman said. "I had a rough 2017, and have been trying to get back on the bike and do something satisfying. Today was that day.
"Huge thanks to the KordaMentha Real Estate-Australia national team for giving me a go, my employer for giving me days off work, and everyone in the team who were so good to me today.”
Chapman asserted her dominance early in the 123.5km stage, overhauling Jade Colligan (Sydney Uni-Staminade) to claim the early mountain classification points.
She was then joined by a larger group ahead of the decisive Myers Creek Road climb where she rode away while collecting the next set of mountain points on offer as the race became a chase to the finish.
A concerted effort from van Vleuten looked to have ended Chapman’s hopes, but she held on to finish with eight seconds to spare.
“I knew that if I went on the first bump, I could make the most of the descent to the second climb,” Chapman said.
“I just went as fast as I could and hoped that I had a team-mate in the chase group. Thankfully I had Tiff Cromwell, who looked after me so well. She gave me the okay heading into the final climb, and I rode away.”
Chapman also leads the mountains classification and picked up the day's most aggressive rider nomination while Hosking leads the sprint classification. Grace Anderson is the best young rider.
Next up for the women’s Herald Sun Tour is a twilight 1.6km time trial in the heart of Melbourne’s CBD.