Not all heirlooms are sold by guild vendors.
Some are sold by faction vendors, some by pvp vendors, some by the heirloom vendor, which is a forsaken in Orgrimmar or a Human in Stormwind.
Some are sold by the medal trader at Boralus or Zuldazar.
And some are sold by random vendors which sells some specific ones.
and some aren’t available from vendors
tentacle trinket and rings come to mind
Dwarf in Ironforge.
I mean I just… like… leveled the professions myself on my alts like a normal person. Maybe invited a friend or two to fill in some gaps so my private guild could have it. Kinda shocked this is even still a thing. Goes to show how bad Blizzard is at keeping the game feeling cohesive by abandoning gameplay ideas and letting it rot. I figured all the heirlooms would be purchasable from a vendor by now.
I could see doing this with a private guild before the heirloom tab system and you had to buy every one you got and professions were actually useful. But these days professions, even when leveling are beyond pointless. I haven’t bothered with them on new characters for years.
And going through all that now just to unlock the pants when you can do it in at most a few hours the way I suggested, is quite frankly masochistic.
a little update: It says to qualify for completion of a profession it needs to have 600 points across expansions. I have a Dreanei jewelcrafter with the levels 310/310, 76/85, 75/85, 75/85, 78/85 which by my calculations makes it 614 points. That didn’t get the completion. Put in a ticket and was told it had to be MAXED out. I had zero problem with gnome engineer though with its bonus.
yeah you took the wording a tad too literally there 
i think the legs were added in MoP?
prior to mop the old (first) looms cost some kind of badges or seal or… idk, but i never farmed the wrath era ones till cata at which point they cost JP.
there was a way back then to convert honor to jp or vice versa, so i remember leveling a couple alts in bgs, capping honor, converting it till i hit jp cap, then capping honor again so i could spend it on either a new heirloom, or instant pvp weapons.
i think the MoP era guild heirlooms always cost gold though while the others cost jp. though now that i type it… my memory seems fuzzier than a pandaren hugging a worgen in a tauren’s lap with a vulpera on his shoulder.
the pvp heirlooms from back in wrath were available with honor or marks…
it took me forever to get them as alliance, because the vendors were in Wintergrasp, and alliance never had control of the zone 
Yeah, leg/cloak/helm looms are third generation loom release (if you count the STV ring as second gen), followed by Garry weapons, then…
Honestly.
It’s a giant mess.
I think next were the admiral rings, with new weapons periodically added in?
It’s also the case that one of the “teleport to capital city” cloaks is tied to a “guild daily quests completed” achievement that only counts out of date dailies and not world quests. Yet my characters continue to get “daily quests completed” achievements when they have only done world quests.
It’s almost as though they deliberately set it up to give the least rewards in different situations regardless of which is which.
Admiral rings and WoD trinkets were added last in HFC patch.
Shoulders, chest, weapons, trinkets and the fishing comp ring were introduced in Wrath.
Helm, cloak and pants were introduced in Cata (via guild vendors)
MoP had weapon heirlooms from Garrosh (flex +) which sadly have being removed unless you got them during SoO.
Garrison Naval rings in WoD, along with the void tendril trinket. Also the trinkets that you could get from mythic dungeons when they got brought in patch 6.2.5
legion brought in heirloom necks.
BFA also brought in new heirlooms as well.
Yup i got 15 points left to do in Engineering and I’ve got it… I picked a god damn Gnome as an engineer. They need to do away with the racial bonuses entirely!
If you WANT to join a guild, you still can; there’s no barrier. But for anyone who does NOT want to join, they are forced to, in order to buy INDIVIDUAL gear, which has 0 relevance to the guild group. I see no justification in forcing people to join guilds.
What a random thread to necro…