This year's European U23 heptathlon and world and European indoor pentathlon champion began her bid for Finland's first medal in Japan with a sixth-place finish in her 100m hurdles heat, but racing in the strongest of three heats, a time of 13.44 seconds was good enough for eighth overall.
She was left frustrated in the high jump, after four first-time clearances, she failed three times over 1.80m - just shy of her personal best of 1.81m - to finish sixth in the second event, losing ground behind the likes of Belgium's Nafi Thiam and the USA's Anna Hall, remaining in eighth overall.
"The high jump was the baseline this season. I felt like I could jump better, as everyone probably says," she told the media afterwards.
Her highest placing in an event so far came in the shot put, where she garnered another 833 points with a best throw of 14.59m to finish third and move up three places in the overall standings to fifth place.
Vanninen then finished fifth in her 200m heat to close out a packed evening session in a time of 24.78 seconds, but the 907 points that earned her would only be good enough to finish 17th of the 22 athletes still in the competition.
The 22-year-old therefore lost the three places she gained in the shot put, to sit eighth on 3777 points with three events to come tomorrow.
"I don't know why 200 metres didn't work. I expected a lot better. If it doesn't work, it doesn't work. Based on the training, my coach Erki Nool and I expected much better."
Leading the way is 2023 World Championships silver medallist Hall on 4154 points, with Kate O'Connor of Ireland sitting second on 3906, then comes Katarina Johnson-Thompson of Great Britain & Northern Ireland (3893).
Just over 100 points off the bronze medal, a podium place is not out of the question for the Tampere youngster, who holds the Finnish record in the long jump - the only event to take place in tomorrow's morning session.
She will need to pull out her best form in that event, to give herself of reaching the medals in the javelin and 800m, which follow in the evening.
