a VLAN can most decidedly ensure QoS
No, by putting QoS on a VLAN, you absolutely do not prioritize voice. You are instead prioritizing the entire VLAN. Broadcasts, DHCP, DNS, SIP, and yead RTP (the actual voice).
A VLAN is 100% never for voice QoS.
Your point that VLAN will not discriminate is noted, but the switch will treat a tagged L2 frame with higher priority than non-VLAN or a lower priority VLAN. As you noted, if a VLAN carries everything, data as well as broadcasts, then within that data will be RTP, etc. So, there you go, a higher QoS is obtained.