The Vestige Patron for Warlocks is one of the most buzz-generating subclasses to come out of Arcana Unleashed. Your patron is a dying god, who manifests as a companion pet that fights alongside you in battle. It's a really interesting inversion of the usual patron/warlock dynamic, and while I think you could certainly flavor it more as your patron manifesting an avatar to serve you, the biggest deal, I think, is that it's a Warlock pet subclass.
As someone whose first class-based RPG was World of Warcraft, Warlocks and Hunters (er, Rangers) have always felt natural as pet classes. And indeed, we'll be able to come with many flavors of pet, as the vestige can be undead, celestial, or fiend, with slightly different abilities depending on your choice.
So, how do we want to build this thing? While Pact of the Blade can be a higher damage option, I think here we're going to go the classic Blastlock route. Admittedly, if you play with Flanking (which appears nowhere in the 5.5 official rules, by the way) you might be inclined more toward Pact of the Blade, but I think my general assumption is that most Warlocks prefer to stay at range.
Unlike other pet subclasses, our companion can actually make ranged attacks as well, so if we want to keep both ourselves and our friend at a safe distance, we'll just hang back and lob attacks from afar.
Because we're building to level 10, we'll be skipping out on the 14th level feature (which is actually pretty great - eventually giving you essentially an extra 9th level slot to cast a Summon spell).
Ok, let's get into it:
As usual, we'll be looking to use a good concentration spell, and one that won't take up our bonus action, as we'll need that for the companion. I'll note as well that we might consider picking up Battle Familiar, which is available to Warlocks and doesn't require concentration. Like the Summon spells we get, it's fire-and-forget. Pact of the Chain is all well and good, but it's usually not worth sacrificing one of your attacks for the familiar's attack (and if we're using Eldritch Blast, we aren't taking the attack action anyway).
At 5th level, our Battle Familiar will be making two attacks that deal 12.5 force damage per hit. We might go with the Flyer to let them dodge in and out of melee range - they only have a 30 foot fly speed, so the safest bet is to fly 15 feet above a target and then dive-bomb them.
But that leaves a concentration slot open. I actually really like the utility that Summon Undead gives, but the math on whether, say, the Putrid one actually poisons and then paralyzes the target can get pretty messy. We might instead choose the Skeletal version to just do some pure damage, and again, at range where we and our vestige will be hanging out.
Essentially, once we have all of this up and running, we can have four creatures (us, our familiar, our skeleton, and our vestige) all bombarding the target with ranged attacks. There are two rounds of set up required, but both Battle Familiar and Summon Undead last an hour, so we could get this running if we know we're going into a fight.
Mind you, we choose one of the Domain Spell lists from the PHB, which function as our patron spells for this subclass. While Crusader's Mantle from the War Domain could be great, our pets aren't using weapons (technically the Battle Familiar would be considered to be making natural weapon attacks, at least in 5.0, but it's a little less clear in 5.5). Thus, I think we stick with these. That's going to take both of our spell slots at level 10, so this is an all-out option, but again, at an hour duration for both spells, we might be able to keep it up through multiple fights.
For invocations, we're obviously grabbing Agonizing Blast. I think that's the only one that's going to directly improve our damage output, but grabbing things like Repelling Blast, maybe even Gift of the Protectors to keep our minions alive, could be good.
Note that if we are casting both Battle Familiar and Summon Undead, we're going to be unable to use Vestige Recovery mid-combat, but again, we're staying at range, so hopefully our buddy isn't going to be getting hit.
Now, feats:
As usual, I'd generally say grab War Caster and then, as a Charisma-based character, we can pick up Inspiring Leader. The tough thing here is that we probably won't be able to buff our Undead and Battle Familiar with this because it takes a short rest, which will eat up the hour that they last.
Anyway, if we're willing to delay capping Charisma, we should have a Charisma of 19 at level 10, for a +4.
So, let's do the damage calculation!
First, our Eldritch Blasts:
We have a +8 to hit, and we deal 1d10+4, or 9.5 average, on each of two hits, adding 1d10, or 5.5 average, on a crit. Thus, against the Wyvern's we hit on a 6 or higher, meaning a 75% hit chance. So, 75% of 9.5 is 7.125 and 5.5x5% is .275, so we get 7.4 average damage per blast, and thus 14.8 from this cantrip.
Next, our Vestige. We use a bonus action to command it to attack once. It also has a +8 to hit, and deals 1d6+7 on a hit (average 10.5) and an extra 3.5 average on a crit. 75%x10.5 is 7.875 and 5%x3.5 is .175, so we get 8.05 from the Vestige.
Then, we have our Battle Familiar, which does 2 attacks that hit for 1d8+8 per hit (12.5 average) and 4.5 extra on a crit. 75%x12.5 is 9.375, and 4.5x5% is .225, so we get 9.6 average per hit, and thus 19.2 average damage per turn from the spell.
Finally, we've got Summon Undead. The skeletal form does 2d4+8 damage on a hit (13 average) and 5 extra average on a crit. 75%x13 is 9.75, and 5x5% is .25, so that's a nice clean 10 damage per attack, and with two attacks, we've got 20.
Add it all up, and we're looking at 62.05 damage per round. Hey, when you're making seven attacks a turn, it adds up!
Now, if we get caught unaware, and still want to commit to this strategy, we basically need to layer things in the following way:
Turn 1: Cast Summon Undead and then bonus action command the Vestige. This means 28.05 damage on the first turn.
Turn 2: Cast Battle Familiar and then bonus action command the Vestige, which brings us to 47.25 damage on the second turn.
Turn 3: Eldritch Blast away and then command the vestige, doing our full 62.05.
Now, the subclass, I think, is mainly giving us utility. That 8.05 extra damage isn't nothing: it brings this up to "really good" instead of just "pretty good" damage. But you can pull this off with other subclasses and do quite well. I'll also say that you might consider Summon Fey instead of Undead if you're going for maximized damage.
Still, it goes to show you that you can put out some big numbers here.