How this is done will vary depending on your router/switches... Most enterprise grade switches have the ability to prioritize data based on the type of traffic, and UCA Softphones are basically SIP softphones, so if you setup your switches to give priority to SIP traffic over other traffic that will likely the best way to implement this.
How many softphones and how large or high-traffic of an environment are we talking here though? If we are talking 5 UCA softphones and an "average" office network with the UCA and MCD server in the same physical location, then switch level QoS is probably not really an issue. Generally QoS issues are at choke points, such as the router between the two VLANs, which would be the real point you need to honor QoS so that queueing of data between the VLANs is handled properly.