:hmm:Why would anyone allow a cat in their home? I swear that a cat's only purpose is to plot pain, anguish and ultimately death on all humans.
You won't believe it, but anti-freeze works awesome.
No, it won't fix the current stain, but a splash in the cat's water bowl will prevent all future stains.
http://www.m.webmd.com/a-to-z-guides/what-causes-cats-vomit
Have you tried different food? Have you tried mixing wet/dry food? Do you gradually transition foods (mix some new/old gradually increasing new) when changing food?
Do you rotate food to prevent food allergies?
http://healthypets.mercola.com/sites/healthypets/archive/2010/11/09/vomitting-pet-cat-health.aspx
