A businessman who had commercial sex with an underage social escort in the high-profile online vice case two years ago was jailed for 12 weeks on Monday.

Nguyen Van On, 50, is the 34th man to be convicted and jailed so far. He was among 51 men charged in 2012 with engaging in paid sex with a 17-year-old girl.

Two weeks ago, Choo Yao Chuen, 38, was also given 12 weeks for a similar offence with the same girl. He paid the escort $450 for her sexual services at Hotel 81 Bencoolen on Oct 1, 2010.

In Nguyen's case, he pleaded guilty to paying $850 for her sexual services at the same hotel on Oct 6 that year.

A district court heard that he chanced upon the website operated by pimp Tang Boon Thiew while surfing Sammy Boy Forum in 2010.

The girl, one of Tang's social escorts, was described as an 18-year-old polytechnic student.

Nguyen wanted to have sex with the girl and sent a text message to Tang to book her services. Tang told him the sexual services would take place at Hotel 81 in Bencoolen Street.

After Nguyen had booked a room on Oct 6 that year, he informed Tang of the room number. Tang then relayed the message to the girl.

Nguyen did not verify the girl's age before engaging in intercourse with her. He paid her $850.

The maximum penalty for the offence is seven years' jail and a fine.