So for backstory one of our players is in the hospital and another is going to be attending a wedding. So for the sake of our hopefully recovering player we are going to do a light filler session unrelated to the current plot. But here's the problem, they are currently in mid attack by a vampire. I had the plan of time stopping and them being pulled into a inter-dimensional carnival but its to early to introduce magic items or other prizes. Any Ideas on what they could do?

Flashback sequence or a complete one off building up some back up characters...

Yeah, if everyone knows it's a one-shot, just use some silly pre-made characters.  Then, when the main campaign allows, bring them to the same carnival and call it foreshadowing.

Flashbacks work well in these situations though the idea of a one shot showing off a certain part of the campaign is a great idea that later comes up in the main campaign 

I suggest the Vampire used a hidden ability to hypnotize the party and sent them through a mass hallucination. They’re swinging swords left and right, when all of a sudden they feel sluggish like trying to run in a dream, and when they give into the inability to move, they realize their surroundings have changed. They look around at each other and don’t recognize the faces they see. 

They’ve been Jumanji’d, and are in different bodies (Will need to issue new character sheets tailored to skills they had and skills their host body has). The vampire has sent them into HIS memories, intent on showing them something that will either rattle them for when they leave the hallucination, or (if he’s been played as the prey instead of the hunter) make them see something that might inspire sympathy for him and get them to stand down when they come out of it.

You can make the session whatever you want, not have to worry about killing one of them, because it’s all a dream. They could face him again in the past (maybe a weaker, younger, version of him due to missing PCs) and defeating him in the dream could be one way to end the hallucination. There could be other milestones that end it, or just running out of time to play for the night could mean you as DM say “Just as you’re getting used to the new body, you feel yourself waking up and see the Vampire exactly where he was before this mind trip. His psychic control of your party seems to have reached its limits.  See you all next time when the real you’s will resume this fight with this Vampire with your own bodies.

When I have half my group missing, I usually run a totally unrelated one-shot adventure with different characters...so do whatever you want! Offer to help pre-make characters if your players need help with it.