The Left voteshare has registered a rise -- from 34,124 votes in 2016 to 41,917 votes in 2017.
The BJP, which is trying to increase its presence in the state, came a distant fourth, polling fewer votes than Social Democratic Party of India.
The victory comes as a booster for the Congress, which is battling accusations of corruption. On the day of Vengara by-polls, Chief Minister Pinarayi Vijayan had announced that a case will be filed against his predecessor Oommen Chandy and other former lawmakers in the Solar Scam on the basis of an investigation by a judicial commission.
Elections became necessary in the Vengara assembly seat - formed after delimitation in 2008 - after sitting legislator PK Kunhalikutty departed for Parliament. The IUML strongman had won the 2016 Assembly polls by a margin of 38,057 votes.
The Left took heart from the increased voteshare. "The UDF may be winning technically but they are failing politically. Their lead in every panchayat has crashed compared to last Assembly election," Kodiyeri Balakrishnan of the CPM told reporters before the results were announced.
K Janachandran Master, the candidate of the BJP, has polled 5,728 votes. In 2016 assembly elections, it won 7,055 votes in this constituency.
While the party says its ongoing 15-day foot march across the state is against the ruling CPM's "murder politics", it is also expected to increase its visibility in the state where the political centrestage is occupied by the Left and Congress-led alliances. The BJP won its first seat in the state last year.
The BJP's voteshare has been less that of the Social Democratic Party of India, the political wing of Popular Front Of India, a hardline Muslim organization. The Centre has been considering a ban on PFI owing to its alleged involvement in forced conversions.