I’m a returning player trying to unlock the Vulpera, but when I go to the war table in front of Baine Bloodhoof, it’s blank so I can’t get the questline started. What am I missing?
If it’s not already, you should see a map of Zandalar.
But if you don’t, look towards the bottom of the interface and click on the tab that says, Scout Map. There should be a (!) on Vol’Dun in order to start the questline. 
The map is there, it is just a blank map with nothing to click to choose a zone.
I’d suggest checking your quest log and seeing if there’s any quest that might lead towards choosing another zone to quest in.
Like doing the War Campaign or, unfinished quests in the other two zones, etc.
Otherwise I’m not sure, I wouldn’t think it would’ve changed for pre-patch but… it’s hard to say.
You also have to be level 50 to finish it.
I’m level 50, but I haven’t played in like a year so I’m fuzzy on where I left off. I’ll look around for any quests in that city I might not have finished.
its possible you may have started it already.
In that one place, Zalandar or whatever. You need to click the map where the princess is, and choose Vuldun and start the achievement line for them. Kinda hard to know exactly where you’re at though. Otherwise check the achievement section and see if you started it.
Can also go to https://www.icy-veins.com/wow/completed-quests-checker and put in your realm & character name and see what quests are still available for the zone
https://www.wowhead.com/list works too, can filter by zone (example: https://cdn.discordapp.com/attachments/679773016703762440/769314265122734110/unknown.png)
Just walk around and say “uwu” and poof, you’re an honorary vulpera.