@Aleluz Presuming this is the old Vegas version, there's a way of selecting a 'Loop Area' , use your mouse to select by clicking & dragging, (the blue area) or double click one event, or double click on an empty space, right click at the top of the timeline for options ....there's lots of details about this tho so you'll have to experiment,

When you come to render, there's a box called Render Options, at the top is an option to Render Loop Region Only, tick this box & it will only render the selected section