The DM will figure that out later

Greyhawk's LG god of war (that the Fighter follows) is Heironeous, who has a rivalry with his LE god of war brother Hextor. One of the cruxes of their schism is that Heironeous was chosen to have aforementioned oil called "meersalm" which effectively gave him mythos-Achilles damage immunity (per AD&D rules, can't be hurt by anything less than +5 weaponry). There's not really rules for the item itself, so I figure 1. it's designed for gods and is only permanent when applied to someone of divinity, and 2. it's incredibly secret knowledge. Heironeous also has a super-secret daughter that was kidnapped by Hextor and given to Dispater to imprison, and there's an order of Heironean paladins whose task is to (in addition to being specialized fiend-killers) find information on how to locate and ultimately free her.

The party had gotten involved with said order of knights, who had been nearby supporting a group of archeologists that had come under pressure by a guild of sorcerers that try to keep all knowledge of Vecna secret so no one abuses his dark power. The knights had lost some of their number on a scouting expedition, in which they had intercepted some travelers that a nearby tribe of Lizardfolk had kidnapped for a Vecnan cult, followed the trail to the cult, and managed to beat MOST of the bad guys but not everyone. The cult's lair is one of few entrances to the underground pathways that lead to Ykrath, Vecna's capital city, long since sunk into the swampy Rushmoors almost immediately after his disappearance following his conflict with Kas (an event which the Ykrathians call "The Downfall"). Legend says that the Ykrathian king (Vecna) was such a feared politician and military commander that it was theorized he somehow had knowledge of all secrets in a mythical dark library. Brief aside, a favorite anecdote of Vecna's cruelty is that when a town's leaders left to offer their own lives be taken to have their citizens spared, Vecna had them strung them up on poles to watch his army murder everyone else.

The endgame is that, after several unrelated cells of his cult set various groups into motion (corrupting a baron in the Fighter's backstory, setting ghouls on the Cleric's monastery home, inspiring a Spirit Naga to enslave a town, etc.), the party starts adventuring, comes into contact with the cult to find the magic mirror he bestowed on them (it individually insta-kills the generals that remain in the city and uses their souls to create an avatar, but the party gave it up to one of the generals and upset the balance of power in Ykrath instead) and informs the order of secret evils deeper in the swamp's underground. The order, having interacted with the archeologists and having the longshot idea that they and the party can find this library through this. The two groups team up, find proof in his library that Hextor allied with Dispater (anything else taken is a pittance of payment, and if Heironean powers suggest taking the legendary ointment for their god's choice of use then even better), and eventually free the daughter (bestowing the oil upon her if they still have it), vastly upending the balance between the two gods of war. As Hextor's might dwindles over the coming years of battle across the region, Vecna (who already has another cult practicing heresy that Hextor and himself are actually the same god) will take some of his evil divine portfolios and domains. A relatively immediate victory for the forces of good, but with a different adversary on the horizon that's become more powerful and has already proven capable of out-maneuvering them.

The only ways out of the plan at this point are if the party either uncharacteristically gives up on the mission entirely/TPKs (in which case the knights at this point still roughly know where they need to crusade to; delaying the inevitable), or somehow convince the god of justice and war to look over giving his own daughter the immunity oil, to instead try to bury the centuries-old hatchet with his brother who had caused so much suffering to the people of the world, but avoiding the power loss entirely. Additionally, even though the mirror likely won't be making him an avatar anytime soon, it's still imprisoning a powerful druid that his cult trapped earlier. Also, the High Magistrate of The Gran March that the party had been knighted by is also the local high priestess of the notoriously-zealous Church of Pholtus (LG god of light and the last organization to fight Vecna's empire in recorded history), who will take the continued existence of Vecna's empire rather poorly and will rile the clergy into a fearmongering frenzy, especially with the knowledge that knights of the realm (party) had willingly agreed to not only pass over having a Pholtan priestess accompany them (they had asked for help earlier, proven useful, and was offered services again if necessary), suffer the great Satan's generals to live, but agreed to export resources to said generals under penalty of subservience (the party saw the subterranean populace that'd turned Gollum-esque over the years underground and took pity). He doesn't have to lift any more fingers at this point, this plan is effectively complete as far as Vecna is concerned, grudges .

TL;DR: Vecna is a manipulative bastard and the majority of the campaign up to this point was completely by his design. That design being I wanted a 1-20 campaign and found a great way to use late-tier-2/early-tier-3 to motivate the party to get roped into extra-evil things later down the line. They find one evil, notice a bigger evil behind it, and in their drive for goodness make enough of a mess for the lesser evil to slink away while they're all distracted.