To many ancient civilizations (most notably Egypt), as well as to alchemists, gold is representative of the sun. As silver was related to the moon. Therefore, it was only fitting that vampires have a severe (ie deadly) allergy to gold. It burns if they touch it. God-forbid it get into their bloodstream.
You may ask, “But what of the daywalkers? Surely, immunity to the sun would equal immunity to gold.” Not necessarily. One of my brothers growing up was allergic to oranges. But he could have orange juice. (I know. Weird right?) Also, look at tomatoes and mushrooms. Both have relatives that are poisonous, yet most humans have no problems eating the former.
Bringing gold into play this way has allowed me to, not only create a new weakness for vampires, but to weaponize the metal for hunters and law enforcement.
And of course, where there’s a poison, there’s an antidote (if you’re lucky). We have anti-gold. Yes, it still burns, but it’s able to combat the poison. Taken in regular doses, it can help develop an immunity to gold. There’s a scene in book 2, Amala gets shot with a gold-threaded arrow. She survives, however another vampire does not. This is because she is used to protecting herself with anti-gold.
Also, gold being such a status symbol, it becomes apparent that there is a sharp class divide between hunters and most other humans. Hunters either have money or benefactors/trainers who do.
Ironically, modern anti-vampire weapons were inspired by designs thought up by vampires during several inter-species wars. But those are stories for different days.