Vadra was asked about the money received by his company as alleged kickbacks in petroleum deals in Gujarat. The agency said the money was used to buy properties in London.
He appeared before the investigators at ED's Jamnagar office in central Delhi following summons.
The case relates to the ownership of 1.9 million pounds of overseas assets and undisclosed entities set up to evade tax.
In a Facebook post before arriving at the ED office, Vadra wrote: "As I make my way for the 13th time, post almost 80 hours with the Enforcement Directorate, answering any number of questions, amidst sensation and unnecessary drama around, I stay focused and calm."
The businessman was upset with news reports and media questions on his health and wrote: "But blatantly and carelessly airing my health matters is not correct... But my time spent with people with worst problems, ailing, blind and orphaned kids to see them still smile, gives me the strength to go on..."
Vadra appeared before the financial probe agency, a day after a Delhi court allowed him to travel to the Netherlands and the US for six weeks of medical treatment.
Last Thursday, Vadra was questioned by the ED for over seven hours on the amount received by his company as kickbacks in petroleum deals in Gujarat. The agency accused him of using the money to buy properties in London.
However, he skipped the ED questioning the next day citing ill-health.
( With inputs from IANS )