The pattern is a schematic description of the vest, and there is no stitch-by-stitch instruction. Any self-striping yarn is suitable for it, no matter how thick (but you will have to recalculate the whole thing).

I give an explanation of how to recalculate the pattern for a different size or gauge. Even though the vest is not perfectly fitted and allows for some freedom in waist, chest or hips, just keep in mind: starting from a certain size, you must have darts or waist shaping. They are not foreseen here, so increase the pattern at your own risk.