Her assertion could be easy to disregard were it not for a 25-minute video investigation posted last month on YouTube by the Russian opposition figure Aleksei A. Navalny, which relies heavily on videos and photographs from Ms. Vashukevich.

She and nine people from Russia, Ukraine and Belarus were arrested late last month in Pattaya, a city about 70 miles south of Bangkok known for its adult entertainment scene. Most of those arrested, including Ms. Vashukevich, 21, who also goes by the name Nastya Rybka, are accused of working without a permit. Some are also accused of not having a valid Thai visa.

Ms. Vashukevich and Alexander Kirillov, the organizer of the sex seminar, spoke to three reporters while standing behind bars and a mesh screen during visiting hours at the Immigration Detention Center in Bangkok. Immigration officials stopped the interviews midway and told the reporters to leave the facility.

Ms. Vashukevich, wearing a bright green T-shirt with the word "detainee" in Thai, is a long way from the days she spent sailing on a yacht with Mr. Deripaska and his friends, including Sergei E. Prikhodko, a deputy prime minister.

According to her version of events, she was working for a modeling agency when she and several other models were sent to spend time on Mr. Deripaska's yacht. She later posted photographs and videos on social media showing Mr. Deripaska and Mr. Prikhodko together on the yacht.

Financial records show that companies controlled by Mr. Manafort owed millions of dollars to Mr. Deripaska. During the 2016 presidential campaign, Mr. Manafort offered to give Mr. Deripaska private briefings on the campaign.

Mr. Navalny charged in his video that Mr. Deripaska's yacht trip was an attempt to bribe Mr. Prikhodko, and that Ms. Vashukevich was one of "several" prostitutes aboard the vessel. In the video, the tycoon and Mr. Prikhodko can be heard discussing Russian-American relations.