There is not a simple answer to your question because it depends upon what type of mRNA vaccine you are intending to develop. Sometimes the RNA is synthesized in vitro with RNA polymerase such as from the T7 or SP6 promoter. Other times the RNA is actually synthesized entirely in vitro on a machine in order to incorporate modified bases which increase their half-life and stability. But there are a number of review articles on RNA vaccines you can read if you search for them.