I'm planning on playing a Paladin/Warlock in an upcoming game and I've picked the Oath of Vengeance, and Great Old One Patron/Pact of Tome. I've made my backstory (island home sunk when invaded by Sahuagin after tribal artifacts (the pact of the tome 'book of shadows' that will be read at a later date), resulting in one wandering vengeance driven Dark Tome possessing dragonborn — think a Lovecraftian twist on Spawn), so I don't want to change the choices, but I'm struggling to see how to make the two sides synergise. I saw "http://rpg.stackexchange.com/q/26469/2788" though, and I might get similar advice.

I've read multiple guides on both, and gone over the powers of them too. Paladins in 5E strike me mostly as Tanks/Defenders and the OoV especially so. They focus on tying down a single enemy, gaining advantage on them and striking them down. Warlocks do have the potential to also deal lots of damage with various riffs off of Eldritch Blast (Agonizing Blast etc). But I don't think you can cast and do a melee attack in the same round, so there's synergy lost there already. Add to the fact that, from what I've seen, none of the Pact abilities (Rituals) nor the warlock spell list complement a defender-esque party role.

I want some way to make these two sides work together, and maximise 'synergy' whilst keeping the theme of Lovecraftian Spawn. If it helps I do like the idea of pinning down enemies with feats like Sentinel, but I don't need a full build, just the best way to synergise.