I seem to remember this was how it worked with a previous expac event, although maybe the vendor was removed eventually.
Is it mentioned somewhere whether the vendor and the primeval essences will persist? Since the currency items do not say “requires:” an event, it should stay, but you can never be sure. And then the question would be for how long they stay.
I will assume both the Premival Essences and the vendors stays after the event, being in line with how the Shadowlands prepatch currency was handled. However, the means of obtaining the currency will be removed.
Yeah, I regret not prioritising to grind out all needed currency during the Shadowlands prepatch two years ago, as I now have nowhere the needed amount of currency to clear the prepatch vendor that still taunts me with his inventory.
There is also a Shadowlands prepatch vendor in The Argent Vanguard, being the Argent Crusade Quartermaster closest to Dalaran. She are on the high ground above the camp tents.
That ‘WotLK’-currency - Argent Commendation, is the Shadowlands prepatch currency. Remember that Shadowlands started with Sylvanas visiting Bolvar the Lich King and breaking his helmet. Thus the control of the scourge got lost, and we had to go all over Azeroth to counter the rogue scourge. This ended in Icecrown where we grinded mobs for the currency.
Also the Shadowlands proper intro - where we go to the Maw, begins with creating that portal on top of the Icecrown Citadel with the parts of the broken helm.
I figured it out because I still had an item from it. Captive Wyrmtongue used to sell stuff for Nethershards. Like Glaive of the Fallen, an upgradeable BoA war glaive.
(And I also remembered that I never played the SL pre event.)