What I've been playing in MODO queues

We're so far from Block being a relevant format, but here's what I've been picking up packs with on MODO:
4 Jund Panorama
4 Grixis Panorama
4 Savage Lands
4 Crumbling Necropolis
1 Forest
3 Island
3 Mountain
3 Swamp

4 Sedraxis Specter
3 Broodmate Dragon

4 Obelisk of Jund
4 Obelisk of Grixis
2 Covenant of Minds
3 Cancel
2 Blightning
4 Jund Charm
4 Resounding Thunder
4 Cruel Ultimatum

SB:
1 Cancel
1 Blightning
4 Agony Warp
4 Vithian Stinger
1 Covenant of Minds
1 Infest
3 Violent Ultimatum

Resounding Scream is a pretty decent SB Card actually. I definitely want to be playing 4 Cruel Ultimatums no matter what. Instead of green you could play white for Ajani Vengeant, Punish Ignorance, etc. Scourglass is even alright against Broodmate Dragon. I like the green though.

What's sideboarding like with this deck?

Against control aka other Cruel Ultimatum decks, take out Jund Charms and some number of Broodmate Dragons for the Cancel, Blightning, Covenant, and some number of Violent Ultimatums, depending on how many counterspells versus troublesome permanents they're playing.

Against creature decks put in the removal spells and if they have big dudes you can add Violent Ultimatums too. Take out some mix of Blightning, Specter, possibly Cancels and/or Covenants.

Honestly it's kind of on the fly at the moment because the metagame is so loose and most decks are just piles of good cards ramping up to Cruel Ultimatum.

Is vithian stinger good enough against the aggro decks?

It's reasonable against Exalted decks and has pretty good synergy with your other sweepers and removal. There aren't many aggro decks.

After playing some with this deck, I've found myself wanting a vindicate effect to handle things that get away. Is there a way to make o-ring work in here?

I mean you could cut green for white, or you can just play Violent Ultimatum and stay green. I assume you want answers for planeswalkers? There are Thunders and Blightnings already.

Yes mostly planeswalkers, but also things like battlegrace angel or a 7/7 thunder-thrash. Violent Ultimatum seems really slow when trying to deal with a Turn 4 Skarkan Vol or Ajani or turn 3 7/7.

Turn 5 Ultimatum happens pretty often, but there's Executioner's Capsule.

I am definitely going to try executioner's capsule. How often does turn 5 Ultimatum happen for you? I usually don't even have green mana by turn 5.

I mean, there's 13 green sources so turn 5 Violent is not really unreasonable. Turn 5 Cruel is good enough too. Or just turn 5 Cancel or removal spell or whatever into turn 6 Ultimatum, that's also often good enough.

The reason I hate to put too many cards in my deck that are just removal spells is that mirror mostly just come down to resolving Cruel Ultimatum so creatures and things that can only kill creatures tend to be pretty crappy there.

Update:
4 Sedraxis Specter
2 Broodmate Dragon

4 Obelisk of Grixis
4 Obelisk of Jund
4 Cruel Ultimatum
3 Violent Ultimatum
4 Jund Charm
4 Resounding Thunder
2 Covenant of Minds
3 Cancel

4 Jund Panorama
4 Grixis Panorama
3 Island
3 Mountain
3 Swamp
1 Forest
4 Savage Lands
4 Crumbling Necropolis

Sideboard:
4 Vithian Stinger
4 Executioner's Capsule
3 Agony Warp
1 Cancel
1 Covenant of Minds
2 Blightning (might become Resounding Screams depending on how draw-go the matchup seems to play out versus how popular planeswalkers are)

Maybe I'm a terrible player, but I find myself not living until turn 6 often enough to think that that's not always good enough. Of course, I've run into almost exclusively aggro decks in the 4-man queues, and I have yet to actually come up against the mirror match.

You never beat aggro decks. Ever.

what cards do these aggro decks play?

Aggro decks that I have lost to:
Nearly Mono-White Bant, With Squire, Sighted-Caste, Sigiled Paladin, and Battlegrace. Game 1 I never found green for Jund Charm, Game 2 the 2nd Battlegrace did me in
Jund Devour-No good answer to 3rd turn Goblin Assault. When he didn't have that he had a turn 3 7/7 thunder thrash. Goblin Assault makes sure that Jund Charm just doesnt do enough. Or he could just resolve Sarkhan Vol.(I've lost 6 straight games to this deck)
Naya Zoo- Turn 1 Nactl, Turn 2 Crasher Swing for 5. Turn 3 Thoctar take 6. I'm at 9 staring down 11 power on turn 4. This is more likely than you think. Or he could just resolve Sarkhan Vol or Ajani.

Executioner's Capsule seems like the solution to the vast majority of those problems. I did beat a Goblin Assault deck though.

Indeed executioners capsule solves a lot of these problems. Main deck or Sideboard? If it's main deck, what comes out, if sideboard, what to swap?

I just have them SB for now, but if Violent Ultimatum is really too slow you can pull or board them and put Capsules main.

Update.

// Lands
4 Savage Lands
4 Jund Panorama
1 Forest
3 Mountain
3 Swamp
3 Island
4 Grixis Panorama
4 Crumbling Necropolis

// Creatures
3 Broodmate Dragon

// Spells
4 Cruel Ultimatum
4 Jund Charm
4 Resounding Thunder
4 Obelisk of Grixis
4 Obelisk of Jund
4 Cancel
2 Covenant of Minds
3 Violent Ultimatum
2 Grixis Charm

// Sideboard
SB: 1 Covenant of Minds
SB: 4 Agony Warp
SB: 4 Resounding Scream
SB: 2 Grixis Charm
SB: 3 Executioner's Capsule
SB: 1 Infest

Ranger of Eos is the best card in the format that's not in this deck. Yes that is a full set of Resounding Screams for the mirror.

Conflux update:

4 Rupture Spire
3 Crumbling Necropolis
4 Arcane Sanctum
5 Island
4 Plains
2 Mountain
3 Swamp
1 Forest

4 Wretched Banquet
3 Tidehollow Sculler
3 Cancel
4 Esper Charm
4 Ajani Vengeant
4 Wall of Reverence
4 Traumatic Visions
3 Broodmate Dragon
3 Martial Coup
2 Cruel Ultimatum

SB:
4 Countersquall
3 Scepter of Fugue
4 Celestial Purge
4 Zombie Outlander

Deck has been pretty good. I'm at least .500 against every deck I've faced. I thought Martial Coup would be better than Cruel Ultimatum due to how many Naya decks there have been, but in the 2-mans there has been much more diversity, so I've wanted the power of Cruel Ultimatum. Wall of Reverence is so insane. I've been thinking about trying some 5c guys like Fusion Elemental or the Mythics. Still a work in progress.