If you've gone nine chapters without noticing they are gone, then the characters may not be that important.
You need to decide if they are important or not.
If they aren't, they could leave in a variety of ways :
0. Excise them completely from the first chapter.
1. the plot of the book is too dangerous, so they went home. "Tangle with vampires? very dangerous. I need to, uh, get some holy water. You go on ahead. I'll catch up." Or alternatively they could bundle the hero's loved ones off to safety. "Get Mom and drive to Fort Lauderdale."
2. They went off to do research, long term planning or long term building. They can show up in the last chapter with a wheelbarrow and a holocaust cloak.
3. They were redshirts and are dead, in a coma, or in rehab, both physical and mental.
