With Marshall still holding her two-point advantage heading into the final round, Vystropova came on strong again but a neat combination in the dying seconds from the Briton helped edge her to an historic win.
"It hasn't even sunk in that I'm going to the Olympics yet. It's crazy," Marshall added.
Leeds boxer Adams started slowly in her bout against Cancan, who she had also lost to Ren in the 2010 final in Bridgetown, and trailed by two points after the first round.
Although Adams enjoyed increasing success with her right hand in the remaining three rounds, Ren scored consistently to stay ahead.
The 29-year-old was unhappy with the judges' verdict and told BBC Sport: "I thought the scoring was a bit wrong, but that's boxing for you. "
I believe I won it but didn't get the decision."
Marshall overawed by World win
The Yorkshire fighter is now aiming for revenge at this summer's London Olympics, having secured her place in the Great Britain Olympic squad by reached the
semi-finals in Qinhuangdao.
"This hasn't dented my confidence. It's been a great experience and I've had an amazing time," added Adams.
"I qualified for the Olympics and that's what I came to do. In London, it'll be on my own turf so the score will be different then.
"I'll be going for gold. I'll get a bit fitter and have a bit more technical ability."
Women's boxing will feature at the Olympics for the first time at London 2012.
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