They told everyone it was cancer.
Unlike in the stories, vampires have all the same weaknesses as humans except for the fact that they don't get ill. If you stab a vampire with a stake, of course they'll die, but a normal knife would work just as well. The only difference with a vampire is that if they're given enough blood in time, they could survive things that would kill a human.
The myths about sunlight and garlic are completely false, as is the rumour about needing to be invited into a house. Put away the holy objects, they don't have any effect other than to make the hunter look like a fool. Scott nearly had an asthma attack because he was laughing so much when he found out that the vampires in Twilight sparkled.
Unfortunately, Guardians did have weaknesses that normal humans didn't. Other than the obvious need for blood, vampires were allergic to Verbena; it had much the same effect on them as werewolves had to Wolfsbane. The other major vulnerability was their bond to their pack. While it made them stronger and was generally classed as a good thing, it was nearly impossible for a vampire to survive if the bond between them and their pack was broken.
The death of the Hale pack caused Amanda to steadily decline in health until it was only her husband and son that kept her clinging onto life. Truthfully, she was ready to give in; the overwhelming feeling of failing her pack and the loss was too much for anyone to bear.
"We're your pack too," Stiles reminded her in a broken voice. He was sat by her bedside while John was at work. "You can't leave us here. I'm not ready to be a Guardian."
"One day," Amanda murmured weakly. "The remaining Hale pack will return and you will become their Guardian." She reached out a hand as grasped her sons' wrist with all the strength she could muster. "You will have a pack to protect."
Stiles let out a startled gasp as images flashed in his mind, too quickly for him to actually see them. He got the vague impression of black hair and blue eyes that changed to red. Under all of the images, he felt an undercurrent of family and belonging, a lot like what he felt when with Scott.
"What about Scott?" Stiles asked with a sob. He knew for certain that there was nothing he could do; his mum was ready to give in. There was a part of him that yearned for the pack he'd caught glimpses of, but the thought of giving Scott up for anyone was too painful to consider. Scott was his only pack, he was his Alpha and Stiles couldn't imagine giving up the loyalty and family that they shared.
"He'll always be part of your pack," Amanda reassured him. "Nothing will change that."
The bond between Stiles and his mum was wavering, even as her voice was getting weaker. Stiles clung a little tighter to her hand, as if he could tether her to the living world. By the time John got back from work, Amanda had already taken her last breath. The sight that greeted him when he walked through the door tore at his heart: Stiles was laid over his mother's body, sobbing into her shoulder while Scott was curled up on the floor, holding a hand to his friends' ankle.
Scott had felt his best friends' pain and loss through their bond and hadn't even hesitated before going over and trying to comfort him. He, too, felt the broken bond that he'd once shared with Amanda, and he was determined not to leave Stiles alone with his grief. He'd learnt what it was like to be Stiles's Alpha, and he was determined to be there for him.
They learnt later that Laura and Derek Hale had survived the fire because they had both been in school at the time. Peter Hale; their uncle had got out of the fire, but he wasn't expected to ever make a full recovery. Laura and Derek left Beacon Hills and their uncle behind, but Stiles had a vague sense that they would return.
He was still weakly connected to all of the remaining pack members, but the only thing the bonds could tell him was that they were alive. He would know them if he saw them face to face, but he would have no chance at finding any of them because the bonds were so thin.
Stiles's father suffered through the breaking of the mate bond he had shared with his wife. It caused him to turn to alcohol more than was healthy, but never enough to cause him to lash out. Even so, Stiles spent most of his time at Scott's house, keeping close to his pack for comfort.
Melissa McCall; Scott's mum, made sure to look out for both boys. Often allowing Stiles to stay the night; it was clear that she was worried about how Stiles and his father were coping. They were at each other's houses so often that both Melissa and John joked that they'd gained an extra son. It got to the point that they didn't have to ask about staying over for a night or two, and they made themselves at home in either house.
Eventually, feeding off of Scott became natural for both of them. Scott was more comfortable offering his blood and Stiles no longer worried that he would make a mistake. As they were still quite young, it took them a while to work out a routine so that they didn't forget. Most of the time, it was Stiles who reminded him, since he was the one who felt the effects, but after a while, Scott began feeling the echoes of his hunger.
Neither of them made any more friends as they grew up; both of them classed as the outcasts of the school. While it was vaguely depressing, it didn't bother them too much since it meant hiding Stiles's secret easier. For the most part, Stiles was like anyone else, his vampirism didn't change much, but little things – like the way he stayed close to Scott or the way he didn't eat much because he only needed to eat food once a week to survive – would be noticed by anyone who paid attention.
Other children his age found Stiles strange. His mother had warned him that that would happen when he came into his inheritance; they could sense something different about him and it unnerved them subconsciously. Scott mentioned that he was perfectly comfortable around him, but they'd known each other before he became a vampire and they were pack.