I suppose that you have a similar Cisco config like this:
ip flow ingress layer2-switched vlan <vlans>
ip flow export layer2-switched vlan <vlans>
This is not possible with sFlow. If you have vlan interfaces (or irb interfaces, it's the same, just another name) on QFX, then you can specify these virtual interfaces. But it is not possible to specify pure vlans for layer2-switched traffic.