Are you sure you really want to do it based upon VLAN? Any QoS plan should ideally take into account not just the relative importance of the traffic, but also how things like latency, jitter and packet poss affect the application. It's often the case that the applications that really need QoS wouldn't necessarily be characterised as the most important. It's also commonly the case that there's a real mixture of time-sensitive applications and applications with different levels of business criticality running over the same VLAN.
