In case anyone is interested, here is the "prompt" (more of outline really) from bagelcat1. I did not quite hit every detail, but she was gracious about that.
The last remaining Vampires are disorganized and distraught because the Alpha is dead. They know from the Alpha's last thoughts that it was Sam who killed their master. A group of vampires (3-5 maybe) trick Sam and Dean (by using a compromised hunter?) and ambush them. When they wake up, Sam is chained to a chair that has been bolted to the floor and Dean is locked into an iron cage nearby. The leader of the vamps monologues that Sam killed their father and hunters like them and the BMOL have decimated their ranks. Without the leadership of the Alpha they are lost and alone and they intend to make Sam pay. Dean is forced to watch as the vamps torture Sam. Finally the lead vamp reveals the ultimate plan. They are going to turn Sam into a vampire. Then he will either kill his brother or turn Dean into a vampire too. Either way the vamps win. Because Dean was once a vampire and shared their collective thoughts, they know about the cure. They also know that it only works until the newly turned vampire feeds and that the cure needs the blood of the vamp who sired the newly turned person. They beat Sam almost to death, (which of course horrifies Dean) then all of the vamps go into another room, returning with a glass of fresh vampire blood. They taunt Dean that the blood will heal Sam and save his life. (Sam doesn't want the blood and even almost unconscious fights back, flashing back to when he was forced to drink demon blood.) Still, the vamps force it down Sam's throat. They put the key to Dean's cage around Sam's neck and then leave. On their way out they tell Dean that they are going to scatter so that even if Dean escapes, there is no way to find them all in time or to figure out which vamp's blood infected Sam.
Of course they underestimate Sam. Eventually the transformation heals him enough that he can free himself. He uses his incredible will power to free his brother and control hisgrowing hunger. Sam wants Dean to kill him before he becomes the monster he was always afraid he would become, but Dean (of course) won't do that. Cas arrives having heard Dean's earlier desperate prayer. They decide that Cas will stay and guard Sam while Dean tracks the vamps. Cas puts Sam to sleep to help ease the burden, but it won't last for very long as his transformation grows.
Dean chases down each vampire. Luckily they were cocky so thefirst few were easy to find. Dean "persuades" one of them to tell him whose blood was given to Sam. It was the leader of course - so Dean tracks the leader who is more crafty than the "lesser" vamps. But it takes days - time Sam doesn't have and Dean is frantic.
Meanwhile, Cas can't keep Sam asleep any more and he is now starving and suffering as the urgent drive to feed makes him almost crazy. Poor Cas has to lock Sam up in the iron cage, but even that might not hold. VampSam would have super strength, but he is weak because he hasn't fed, but Cas worries about the effect of the vamp infection on his friend and what it will do to Dean if they can't save Sam. At one point Sam begs Cas to kill him, to save Dean from having to do it, but Cas can't do that to his friend. Everything rests on Dean!
Finally Dean finds the lead vampire who suggests that the lesser vampire lied and denies that Sam was fed his blood. He taunts Dean and tries to make him doubt. If it isn't this vampire's blood and they try to feed it as part of the cure to Sam, then Sam will become a full vampire and Dean will have to kill him. It's a gamble. The vamp tries to convince Dean that it's too late that there is no way Sam could have resisted this long. But Dean knows that Sam is the strongest person he knows and that if anyone can resist it will be his brother.
Dean kills the vampire anyway and takes the blood back to Sam. Cas has to hold Sam down with his angel strength, but they force the cure down Sam's throat. It takes a long time to start working and Dean despairs and thinks about ending them both. He was in such a dark place at the beginning of season 13, and even with Cas back, he's lost a lot of his faith. Without Sam he doesn't think he can keep going. His logic is that Mom is dead (he doesn't yet know that Mary is alive) Lucifer is gone and Cas can find and take care of Jack. Eventually Sam turns a corner and begins to recover. There are a few tender moments of relief between the brothers. Once he is fine again, Sam asks what Dean would have done if it didn't work. Privately Sam would have wanted Dean to kill him. Privately Dean would have let Sam kill or turn him rather than kill his brother. But out loud they lie to each other.