only one attack is replaced.

Vecna casts dimension door to teleport directly to the dragon. He then uses vile teleport (9 damage) and regains 80 hitpoints. He then makes two attacks with afterthought.

to hit: 33 damage: 6 plus 11 and take 2d8 more necrotic damage at the start of your turn, you can then make a DC 20 con save to end the effect. (It happens every turn)

to hit: 30 damage: 6 plus 4 and take 2d8 more necrotic damage at the start of your turn, you can then make a DC 20 con save to end the effect. (It happens every turn)

Con saves 

20 or 29
20 or 27

Legendary Action: Wing Attack 
DEX Save 25 or go prone and take additional 17 damage

Vecna: 198 hp (was reduced by fall, then vile teleport healed the 80 fire damage from the breath)
Dragon: 349/385 hp, 2/3 legendary resistances

Just gonna post that again

Con saves 

30 or 23
35 or 34

Legendary Action: Wing Attack 
DEX Save 25 or go prone and take additional 13 damage

Vecna: 198 hp (was reduced by fall, then vile teleport healed the 80 fire damage from the breath)
Dragon: 349/385 hp, 2/3 legendary resistances

legendary resist wing attack.

breath recharge: 2

The dragon, seeing the liche still flies lashes out with a bite of jaws.

Attack: 33 Damage: 19

After that it rears it claws in an attempt to shove the liche prone and out of the sky.

STR check 1: 24 or 19
STR check 2: 27 or 28

If the liche falls, it then flies down 30ft. (170ft from liche)

First is an autofail, so legendary resist.

19

if failure, legendary resist.

On my turn, vile teleport to deal 10 damage and regain 80 hp, and then dimension straight to the ground.

With the shove prone, I think it's a contested skill check as opposed to a saving throw.

Well that’s an autofail, so i fall down. How high up are we?

Quote from TheFriendlyArchfey >>

Well that’s an autofail, so i fall down. How high up are we?

 I believe Vecna had Dimension-doored to the dragon and then attacked with afterthought which has a range of 5ft. As the dragon was 200ft up, that would put Vecna 195ft up.

Okay, so i take 63 bludgeoning, halved cause resistance.

I believe you also make concentration saves on [Tooltip Not Found].

15

I believe that’s a fail, so I will prepare to use rotten fate if you fly in range.