Vinculum not "seeing" through USB hub

I'm using a FTDI Vinculum VDPS (disk, PC, slave) USB controller.
I can access and control devices plugged directly into the Vinculum USB port. When enumerated with QueryDevice they appear as device 0. I select them with SelectDevice 0 and everything works as expected.
When I try with devices plugged in through a hub, the hub appears as device 0 and the other devices appear as devices 1+. However, trying to communicate with them fails. I select them with SelectDevice n, which succeeds, but then, eg, DeviceReadData gives Command Failed.
Is this something people have come across? Am I missing something? I've tested the devices and hub plugged into my Windows PC and everything works as expected.

Some details:
Keyboard plugged directly into USB port.
Send SCS,13,IPH,13,QP2,13,SC,32,0,13 ; ShortCommandSet, IOHex, QueryPort2, SelectCurrent 0
'Ok' replies to all
Send DRD,13 ; DeviceReadData
8-byte keyboard packet returned.

Keyboard plugged into hub plugged into USB port.
Send QD,32,1,13
Returns 32-byte packet saying 'keyboard' and 'device 1'
Send SCS,13,IPH,13,QP2,13,SC,32,1,13 ; SelectCurrent 1
'Ok' replies to all
Send DRD,13 ; DeviceReadData
Responds 'Command failed'

Some speculative purchases and more testing reveals a few things.

I bought a second-hand Dell keyboard which had a card reader on it. This shows up in my USBDevices program as HID (Keyboard) and UKN (Other). It works directly, and works through a hub.
On a hunch I plugged my first two keyboards into a USB-PS2 plug, and they worked plugged into a PS2 keyboard socket on my PC. The Dell did not. This rang a bell.

Those keyboards that have a USB plug but can be plugged via an adaptor into a PS2 socket are actually PS2 keyboards that detect if thay are plugged into a USB socket and masquarade a stripped-down USB protocol. A faint memory prompts that they only work as USB keyboards if plugged directly into the USB master as they aren't actually USB keyboards, they are PS2 keyboards with a USB plug.

The Dell keyboard is a USB keyboard as it has a card reader, and only USB devices can provide more than one input device.

So, the solution to the problem is: ensure the USB device is actually a USB device and not a non-USB device with a USB plug.