On page 12 of the Buffy comic Double Cross (found in graphic novel Food Chain, or BtVS comic issue 20) Angel is being strangled by the demon. He passes out, and so enters the strange dream world with Buffy. HOWEVER how could strangling have an effect? At first you may think that the demon's grip was so strong that he may have passed out with the pain or nearly being decapitated, but in his though bubble he says 'Grip...so tight...losing air...' which makes no sense. He is definitely still a vampire as he goes into vamp mode before the demon tries to strangle him, and vamps don't need air...