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Can never remember the "right" name for the faction keyword, so I think of it as Space Marine (Adeptus Astara or something). Is it still a legitimate keyword for detachments...so I could have a unit of LoTD, an Apothecary, Captain, and Ancient and call it a Vanguard?

It is, and you could. But the Apothecary, Captain and Ancient would give up their chapter tactic.

Its not 100% clear what they ment as it's clear a detachment containing even a single unit of LoD gains 0 CP, but not exactly how they would interact with non LoD units.
‘Saviours From Beyond: As long as your Warlord is from the Imperium, you can include this unit in a Vanguard
Detachment even if that Detachment contains no HQ units. However, if you do so, that Detachment’s Command
Benefits are changed to ‘None’.’