Jan,
What application are you using to query the OID strings?
Using NMS/MIBTools and querying 1.3.6.1.2.1.17.7.1.4 does indeed respond with VLAN information.
NOTE: entPhysicalSerialNum which is OID 1.3.6.1.2.1.47.1.1.1.1.11 is show an S-4 Chassis and not a Securestack.
Note: Test on an SSA (running 8.62.02.0022) show the same MIBs are being populated with VLAN information
Thanks,
John Martineau