Being inspired by the Vanguard Spiker, after many hours with a Ritualist and after hundreds of different PvE builds, after obtaining the Legendary Vanquisher-title, I've come to the conclusion, that this very build here, has to be the ultimate build for Vanquishing with a Ritualist.
The Build
[Lily's AP;OAej4xiM5Qyl2kLQ7iLTXOMTJXA]
The Description
Inverting enemy's damage output, dealing direct 80dmg, causing a knockdown, having a kd-assassin to keep the enemy down, energy management, burning and resurrect, you're all set. You can customize the build further, but having 3-4 of 5e skills is recommended. Just make sure you time your 13sec Assassin's Promise well and you're all good. Some enemies need different style of approach so you just have to choose wisely which one of the 4 first skills you're up to use first if you want to be the most effective. The 2-4 last skills should be used as energy management, because AP's 17 energy won't cover up all the energy usage for the first 4 skills.
The Replacements
Replace [Spirit Burn] with [Splinter Weapon] or [Ancestor's Rage], if you want to deal damage non-directly and you have a friend with a melee profession. When you know the area has heavy healing, take [Warmonger's Weapon] with you and slap it to a melee ally if possible and replace [Pain Inverter] with ["Finish Him!"].
I just love to take out the bosses with PI, although "Finish Him!" deals 80dmg directly and inflicts 2 conditions at the same time. The timing with "Finish Him!" might be hard, because enemies' healthbars happen to go down so fast

[Protective Was Kaolai] might be a good choice too, if you don't have enough healing already by your team. (I still prefer the all-offensive-style with this build.)
Edit: (credit to Joseph Spiritmaster)
[Spirit's Gift], [Explosive Growth] and [Boon of Creation] (with one spirit if your heroes/allies don't have one) might be also good choices, if you don't mind relying on enchantments too much

The Team
You don't have to have heroes for this to work, but to be the most effective..
Add a group of Discord as your Heroes and you're all set.
The Last Tips
For the hardest Vanquish-areas like Tasca's Demise, Ice Floe and such, ask your friend to accompany you with another set of Discord.
If you're running the PI variation of this build, you can ask your friend to take the "Finish Him!"-version so you don't accidentially cast PI to the same enemy your friend has.
The counters this build has are normal anti-caster: Dazed, Knockdown, Interrupts and energy denial.
The good thing here is, [Power Block] can't shutdown you for good.
The Last Word
Have fun vanquishing and C&C always welcome ^^!