Struggle against Nurgle deamons, need help.

Yeah. I hate WoC and DoC for exactly those reasons. They were GW's favorite army so they made them uber powerful. I'd even say broken. People like to point out that they're expensive and don't have shooting, but LOTS of armies are expensive (Dwarfs, any Elf race, etc.) and other armies (VC included) don't have shooting either (not even a Hellcannon), but none of those are close to being as strong. So yeah. They're broken IMO.

In fact, my local gaming group has written off those two armies, because you have to ban SO MUCH from their armies JUST to have a fair game. It's not fun.

Sorry, as far as what you can do, the only thing I can suggest is to spam the heck out of low-level necromancers and always opt for Invocate as well as take several Mortis Engines with the Blasphemous Tome upgrade. Then take so many skeletons and Zombies, that they'll never cut through all of them. In your magic phases, throw 1-2 dice at Invocate with all of your Necros. The Mortis Engines will help with this. Theoretically, he won't have enough dice to stop all of the spells. Also take the Black Parapet to maximize your power pool. If you have a Terrorgheist or two, then while they are tied up with all those Skeletons and Zombies that keep coming back, Scream the heck outta the weakest units. Get those Victory Points. The rest, just hold up for the entire game. It's boring as hell, but if they're going to bring a broken army, you gotta do what you gotta do.

I wouldn't try taking units like Crypt Ghouls, Vargheists, or other hammers, since they're not going to be effective at killing them. They'll just go down easily and give him VPs. You could try taking a Blender Vampire Lord on Zombie Dragon, if you want that character who can punch a hole in something, but that's just up to your personal preference. I'd probably also leave the Grave Guard at home, since they will get killed by the dozens and are very expensive and could hand him the game with those VPs. A couple of units of Hexwraiths might be a good choice though. If he can't touch them, then he can't get those VPs. Just stay away from characters, or creatures with magical attacks. Make sure you know which is which before you start your game.

Hope that helps a bit.

Lore of Death to snipe characters - nurgle doesn't close particularly fast and your vampire characters have high leadership.

You don't kill nurgle daemons in hand to hand by direct casualties. You hope to static combat resolution them down. Bricks of skeletons are relatively cheaper (and replenishable) get in the flanks and the front with stuff simultaneously. Use Direwolves to redirect other units from the combat. With no panic checks you can redirect for as long as you have dogs/bats etc to keep getting in their way.

Nurgle is a slower moving list in general. Your Blood knights should only be charging things that you want them to fight. The unkillable block of beasts of nurgle is not what you want your hammer to hit (hammer against anvil does not hurt the anvil and will eventually break the hammer.) Maybe in the flank/rear of a brick of plaguebearers to put some active combat resolution wounds into the mix.

Here's the thing -  chaos in general is really good at hand to hand. You aren't going to beat them trying to outmuscle them in combat with most WFB units. But there is a reason they weren't at the GT level generally highly rated armies in 8th ed. They didn't have the tools to deal with alternate situations then someone willing to punch face to face.

Sit at max range for your damage dealing spells have some dogs ready to tie him up as he gets close, and move some fast moving stuff along the flanks to try and get around the back. Set up for a turn 3-4 combo charge with lots of your points against less of his while tying up the rest from a prolonged combat (which is likely to happen with these two armies)

Edit also should mention Purple sun. While a miscast has a small chance of being fatal for you anything else is a lot of casualties and power dice generation in your favor.   

The ring with Vanhels would be useful in this instance, as you would potentially be able to cast the spell twice - assuming the first is dispelled.

If you want to stay away from cheese (more than I can say for your opponent) I do think ghoul hordes are effective against plague bearers, providing they have support. Okay they hit on 5's, but they are poisoned attacks, you would get 40, and with the previously mentioned corpse cart (and T4) they should match them point for point. In fact a corpse cart with a lv1 necromancer equipped with book of Arkhan or perhaps Staff of Damnation for 10 additional attacks and invocation would be a pretty good combo in this case. Crypt horrors are similar for the same reason. Including mortis engines as well would mean that they too have 4+ regen.

Failing that, skeletons, full command, 2 vamps perhaps with Curse of the revenant and Skabscrath, one with Aura of Dark majesty 5 wide, with 4 Banshees in 2nd rank together with 2 terrorgheists or 2 Engines and throw the cheese back in his face 😝.

Well remember he has some damage causing spells himself so solo mages can be a risk.

The benefit of Hexwraiths against Nurgle is that they are flaming, and if he is pimping his regen save it could be useful in that regard. The same goes for the Road of Flaming Death. I would also be tempted to give Skabscrath a go, it not only grants Flaming Attacks and other bonuses but also gives you Death Shriek which could work nicely on low Ld Daemons. Banshees & Terrorgheists could also be beneficial although as others have mentioned, Banshees lose some application through Ethereal being useless though. I would be tempted a bit by the Black Coach too, can easily access Flaming Attacks.

I think you need to go for a balance between tarpits and a few hammer units (Blood Knights, Varghulfs, GG/Black Knights) who would generate your combat res as gjnoronh suggested.

Lore of Shadow is also a good option since it penalises low Initiative and Nurgle I sucks. It would probably be my choice for my main caster, practically every spell would be pretty beneficial to you with Pit of Shades being particularly epic

You should also use a Zombie Dragon just because. That way noone ever hits anyone in combat

Outmanoeuvre and snipe with 2 terrorgeists, Ghoul King mounted on terrorgeist with Skabscrath, Curse of revenant and dark majesty. 4 shrieks, no combat 😋.

Perhap snipe is the wrong word here...

But there is sooooo much that's broken and utterly fun
WM are broken, at the moment I play 4 every game but still lose as I can't finish the units off