The location of vents associated with an eruption is very complex. Vents locations are controlled by the detailed structure of the earth they are found in. If cracks are present, rising magma will exploit these natural weakness zones and rise to the surface. During an eruption, sometimes one vent shuts down and another one opens up. This has to do with the balance of pressure and gravity forces as well as the rate of supply of the magma from below. Magma that resides deep within the crust will also deform the surface of the earth-bowing it upwards like a balloon inflated with air. This can cause new cracks to form and these new cracks can become pathways that magma follows to the surface. So your question is a good one for which there is no "simple" answer. Like people, each volcano is an individual and is unique in certain respects.
I hope this answers your question. There are many places on the WEB and in other books where you can read about volcanic vents and lava flows.