My guess is that non-vampire objects simply do not move through the air fast enough or undetectably enough to hit one that's still moving around.
Bullets? Normally they wouldn't be much use but incendiary rounds with a sufficient firing rate should be effective in the space of time when the opponent is closing. Their closer proximity and forward progress makes it unlikely that they would be able to dodge. Especially against a team of 4 standing side by side. Recent developments in technology like that is something that is unlikely to have been fully investigated and probably has potential uses that have not been considered due to how useless guns used to be.
So when they fight, they get up close and personal, and they bite and crush and tear.
Not even melee weapons? For example, swords or even heavy gauntlets made of tungsten carbide? Anything to add weight to a strike. Relative to their strength vampires are incredibly light and the extra kinetic energy would make a huge difference.
Vampires literally limiting themselves to biting, crushing and tearing with only their bodies as weapons is better explained by a combination of narrative appeal, animal instincts and tradition than by considered evaluation of optimal tactics.
Incidentally, just how durable is a vampire? Obviously they are able to damage each other and wolves and werewolves seem to be able to kill them from time to time. Even Carlisle's father apparently hunted them. Where do they fit in relative to, well, any material known to man?
It doesn't seem that vampires are a great deal more flammable than, say, humans. I'm not sure if an incendiary round would catch.
i will, at any rate, not be including guns in the story even if it makes sense, because I really, really, really hate them and gosh darn it I am doing this because it's fun.
Swords would probably break - really durable gauntlets have potential, although I've got enough other things to throw at Bella that she's not going to have a chance to get down to weapons research for at least a while.
It's not clear exactly how durable a vamp...
In the vein of the Harry Potter and the Methods of Rationality discussion threads this is the place to discuss anything relating to Alicorn's Twilight fanfic Luminosity. The fanfic is also archived on Alicorn's own website <strike>(warning: white text on black background)</strike>.
Previous discussion is hidden so deeply within the first Methods of Rationality thread that it's difficult to find even if you already know it exists.
Similar to how Eliezer's fanfic popularizes material from his sequences Alicorn is using the insights from her Luminosity sequence.
Spoilers for the fanfic itself as well as the original novels need and should not be hidden, but spoiler protection still applies for any other works of fiction, except for Harry Potter and the Methods of Rationality chapters more than a week old so we can freely discuss similarities and differences.
EDIT: Post-ginormous-spoiler discussion should go to the second thread. (If you have any doubt on whether you have reached the spoiler in question you have not.)