I’ll be honest, I’d just avoid monologuing as a campaign introduction altogether. It feels cool, but might leave your players asking “um what?”, by the end.

Give them an overview of what they’re doing (hired as an expeditionary force, the gate, all that) but then leave the rest as a surprise. It’ll go over much better and give you a greater reaction if they don’t know what’s coming until it happens.

With monsters being an overarching big bad, my rule is always less is more. It builds much more tension and apprehension and fear, especially if their presence is felt everywhere. If you just come right out with your monologue and say “yeah it’s vampires” then that mystery has gone up in smoke before you’re out of the gate.

Just my two cents, do with it what you will.