The 28-year old Vlasic, who holds the second best jump of all time with 2.08 metres, has had surgery on her left ankle and heel this year, but the recovery was complicated by a prolonged bacterial infection.
"This whole season, particularly London, is in question. I will make my decision a day before the Olympics," Vlasic said.
Vlasic has had a history of health problems, culminating in thyroid surgery which sidelined her for most of 2005.
She recovered to win gold at the 2007 world championships and she took silver at the 2008 Olympics in Beijing.
Vlasic, world champion again in 2009, finished second at last year's world championships in South Korea after struggling with a knee injury and poor form for most of the season.
"I don't want to go to London and jump 1.90 or 1.95. Everyone knows what I want in London.
"I don't want to be fearing whether I'll make it to the finals or not. That would be devastating."