Transcription of the cloned gene is a necessary component of the vector when expression of the gene is required: one gene may be amplified through transcription to generate multiple copies of mRNAs, the template on which protein may be produced through translation. A larger number of mRNAs would express a greater amount of protein, and how many copies of mRNA are generated depends on the promoter used in the vector. The expression may be constitutive, meaning that the protein is produced constantly in the background, or it may be inducible whereby the protein is expressed only under certain condition, for example when a chemical inducer is added. These two different types of expression depend on the types of promoter and operator used.
Viral promoters are often used for constitutive expression in plasmids and in viral vectors because they normally force constant transcription in many cell lines and types reliably. Inducible expression depends on promoters that respond to the induction conditions: for example, the murine mammary tumor virus promoter only initiates transcription after dexamethasone application and the Drosophila heat shock promoter only initiates after high temperatures.
Some vectors are designed for transcription only, for example for in vitro mRNA production.  These vectors are called transcription vectors.  They may lack the sequences necessary for polyadenylation and termination, therefore may not be used for protein production.