Ticket 925314. The first suggestion was to enable extended VLAN information in the NCM Inventory settings. That didn't work. Then the suggestion was to upgrade to NCM 7.4 and to apply Orion platform 2015.1.2 Hotfix 6.That didn't work. It was only then that I got the answer that "it was mentioned that NCM doesn't collect VLAN info for mentioned devices for now. I was advice [sic] to feel free to submit feature request."
So that's where this ticket will be closed. TL;DR: Juniper and Dell devices are not supported to the same level as Cisco in NCM. Not that that is news to anyone