Think about whether this is just new or if he's been acting differently before this morning. Has he been drinking and using the litter box as usual, etc...prior to this vomiting. Did he get into anything like cleaning supplies?
Try to keep him encouraged. Don't act too alarmed around him. Keep him in and quiet and watch his litter box and food/water intake. Encourage him to drink water.
Often a vet's office will have a voice recording telling people who to call in case of emergency. If yours doesn't call another vet until you find one who does have that information on their voice mail. Usually they all do.
Unfortunately it is pretty expensive to go after hours but if you are concerned, do go for it. Let us know how it turns out. Hugs to you both.