Okay, so here's the outlined rough idea of a mental crossover that may or may not work.
So a vampire bites two kids in Sunnydale, big whoop it happens. But kid 2 is from Albuquerque, New Mexico and was getting high with his friend. The vampire now has a taste and an addiction for this drugged blood. Uses thralls & sycophants to track down the drug.
Sunnydale soon has a unique drug problem. Buffy and gang take on methpires (tm K Curley and probably said by Xander) who are harder to kill than ever before. Walt White wonders why there is a massive spike in product and comes to Sunnydale to find out; he meets Giles and they hit it off being teachers.
Soon Buffy & Co defeat the vamps but need to cut off the drug supply so head to New Mexico. This puts Buffy directly at odds with Walter White who tries to get Jesse to kill the teenager.
Hijinks, battles and reveals, Walter manages to capture Willow or maybe Xander and holds them hostage. Buffy takes Jesse hostage and stalemate. Terms are agreed: no more drugs to Sunnydale, no Slayer in New Mexico. It's a bad deal but agreed on by all.
Epilogue: a couple of human teens are taking drugs in Sunnydale High much to Buffy's dismay & disgust (insert possible school special on drugs) whilst in New Mexico Jesse and Walter have a rare beer together and see a band take the stage called Dingoes Ate My Baby (Oz the werewolf has come to keep an eye on them)