Local media had earlier reported Villavicencio had been shot at a campaign event in the capital Quito on Wednesday.
"For his memory and his fight, I assure you that this crime will not remain unpunished," Lasso said on X, the social media platform formerly known as Twitter.
"Organised crime have gone very far, but all the weight of the law will fall on them."
Lasso said he would host top security officials at an urgent meeting.
Videos on social media purportedly from the campaign event showed people taking cover and screaming as gunfire sounded.
According to opinion polls, Villavicencio's support was at 7.5%, ranking him fifth out of eight presidential candidates for the August 20 vote.
Villavicencio, from the Andean province of Chimborazo, was a former union member at state oil company Petroecuador and later a journalist who denounced alleged millions in oil contract losses.
He had on Tuesday made a report to the attorney general's office about an oil business, but no further details of his report were made public.
Villavicencio was an outspoken critic of former President Rafael Correa and was sentenced to 18 months in prison for defamation over statements made against the former leader.
He fled to Indigenous territory within Ecuador and later was given asylum in Peru.
As a legislator, Villavicencio was criticised by opposition politicians for obstructing an impeachment process this year against Lasso, which lead the latter to call the early elections.
"Today more than ever, the need to act with a strong hand against crime is reiterated. May God have him in his glory," fellow presidential hopeful Jan Topic said in a post on X.
