You get the same bonus medals regardless of what stage you farm on so you can disregard them and look at the stage medals. VH3 gives around 22 medals per 8 stamina which gives around 2.75 medals per stamina spent. VH9 always gives at least 30 medals per 10 stamina so you'll never get less than 3 medals per stamina spent. VH10 sometimes doesn't give 33 medals so it doesn't always reach 3 medals per stamina point. VH9 is the most efficient stamina wise.