"Concern has been increasing, because we are quite a number of months with inflation below 1 percent and that is certainly a problem," Vitor Constancio told CNBC from the Ambrosetti Forum in Italy.
"We are really concerned with the regime of low inflation."
The ECB opted to hold its key interest rate at 0.25 percent on Thursday, as well as keep the rate on its deposit facility at zero.
This was despite calls from the likes of the International Monetary Fund for the bank to combat rapidly falling inflation and keep the recovery on track. This week official data showed that euro zone inflation slipped to a 52-month low of 0.5 percent in March.
Constancio said the latest data had not altered the central bank's forecast of 1 percent inflation over the course of 2014, but that it was eyeing the situation closely.
"The expectation is we are now seeing a sort of bottom (in inflation), but we have to be sure of that," he said. The ECB aims to maintain the inflation rate at just below 2 percent.
Constancio also repeated Draghi's message Thursday that the ECB's Governing Council was agreed in its commitment to use methods beyond the conventional interest rates to stimulate the euro zone's economy.
The vice-president confirmed that a ECB bond-buying, or quantitative-easing (QE), program had been discussed, although not in detail.
"We did not discuss details (of QE) because that was not the environment to do it," he said. "We have had some discussions (in the past) and we will have more, about the details... of course, the details are important."
Draghi has repeatedly said there are other policy tools the ECB could use -- and some of the more unconventional measures have recently garnered attention.
Ahead of April's monetary policy decision, Germany's Bundesbank President Jens Weidmann stoked speculation by saying QE had not been ruled out. Unlike central banks in the U.K., U.S. and elsewhere, the ECB has not launched a bond-buying program. Such a move would be a controversial shift in policy and fraught with difficulty as there is no single euro zone bond and the ECB is prohibited from helping out individual governments.