system
Given the following post, respond as if you're commenting on Reddit:
human
[Skyrim] Why is the dawnguards weapon of choice against vampires the crossbow?

Like, I understand wanting to keep distance from the vampire, but it seems like it'd be easier to just use a normal bow instead of basically inventing(I actually dont know if the dawnguard invented the crossbow Tes lore is not my strong suit) a new weapon which requires new ammo not sold anywhere. Plus, a big appeal of the crossbow is ignoring armor, but vampire armor is more form over function so that point seems moot too.
gpt
The dawnguard didn't invent the crossbow. They perfected it. The main appeal of using it over a bow is stopping power and minimum range.
 A bolt can punch through plate armor fairly easily and requires almost no training beyond point and loose. Against a vampire it can knock them off their feet and back against a wall or through any hardened hide and armor they may be using. Really the only thing that stops a bolt is a wall or gravity. 
And the second reason they prefer crossbows is how closely they can used in combat. It's the difference between a long barrel rifle and a pistol by comparison. You can use a crossbow in a tomb or tight space that a bow just isn't going to cut it in. A skilled archer can use a bow at point blank range and knock and draw an arrow in seconds. An unskilled crossbowmen can do the same. And the point of the crossbow is to merely stun or pin the vampire long enough for a hammer or sword to pierce or behead the monster.