Having experienced this in the new bimmer 2 days after i got it,
Remove the seat bench from the car – they all lift of relatively easy (unless hinged).
Proceed to see how much puke you missed as it slid down the seat belt and bench.
Mop up puke
Lift any wires out the way.
Get Bicarb, make paste and rub it everywhere including seat belt (this is a must they seem to suck it all up)
Get Seat Bench, and appply mucho bicarb paste everywhere – leave to dry
Once dry, hoover everything up.
Then – you need to wash the seat bench and belts try to soak them in a bucket (seat belt) as you will get white staining from the bicarb – don’t worry it comes off.
then get some biological washing stuff – vanish is pretty good – but make sure it’s not the one with chlorine/bleach. give everything a good scrub.
Rinse everything a few times, then allow to dry.
If leather – get some leather conditioner (cream) apply liberally, then buff off.
This worked 100% in my car.
oh, also, try if you can to get inside the seat belt buckle/latch – puke is good at hiding.