Lookup VLAN tags in sub-dissector

Hi,

I am currently writing a dissector which handles a registered ethertype, and I have a requirement to reassemble a subtype from multiple fragments. The key/uniqueness for these fragments is held in the vlan tag/ID and so I need to access this information from a sub-dissector.

I think I am missing something fundamental, but some guidance on how to access the vlan information would be great. Currently I am looking at the column info for QinQ VLAN tags, but I assume this is a hack and not an appropriate way to ascertain this information. Perhaps tapping or some other construct is required?

Regards

John