NDIS 6.0 backward compatibility problem

We have a fully functional NDIS 5.1 Miniport driver which was thoroughly tested
and is working perfectly under Windows XP. Recently we updated the .INF file and
successfully installed the same driver under Windows VISTA. We did not change
anything in the driver's source code hoping it will run correctly under VISTA.
However we encountered the following problems:

1. From the DriverEntry() we call NdisMRegisterMiniport() with MajorNdisVersion = 5
and MinorNdisVersion = 1. After the DriverEntry() and the InitializeHandler are
called we start getting OIDs which are specific to NDIS 6.0 and since we register
as NDIS 5.1, we do not treat those OIDs correctly.

2. We are NOT getting the OIDs we used to get: like OID_802_11_BSSID_LIST_SCAN and OID_802_11_BSSID_LIST
As we used to with NDIS 5.1 under Windows XP.

Is it possible in some cases NDIS 6.0 is not fully backward compatible with NDIS 5.1?
Is it possible VISTA mistakenly "thinks" our driver is NDIS 6.0 instead of NDIS 5.1?
Are we required to apply any minor changes to our source code?

Regards,
Dmitry.