It doesn't. The vaccine is a piece of mRNA (in this case part of the spike protein) which has a cap on one end to stop degradation from the start of the messag

It doesn't. The vaccine is a piece of mRNA (in this case part of the spike protein) which has a cap on one end to stop degradation from the start of the message (which would garble it completely). This piece of RNA is not replicated. Whereas the virus encodes the whole spike, the capsid protein and whatever else the virus needs to replicate like promoter sequences and an origin of replication. It is much larger and is replicate. Basically it skips all the viral machinery and just codes for a small piece that identifies the virus to the immune system.