The vector used to transmit a gene segment is a

Plasmid

Explanation:

A plasmid is a small-molecule of DNA that can replicate independently in the cell. They are present in bacteria ( some archea) but its does not have the essential information of the bacteria for life. Plasmids usually carry ganes that benefit the bacteria like antibiotic-resistant genes that can be transferred to one bacteria to another.  

In molecular biology, plasmids, artificially constructed, are used as vectors to transfer a specific characteristic to another cell. They are usually used to express particular genes of interest, like the gene that codifies insulin is transferred in a plasmid to a bacterial cell, to produce insulin for farmaceutical industries.