Young animals are much more suceptable to the toxic effects of all drugs (just like babies). The advice that vet gave is atrocious, peroxide will cause all sorts of tissue damage to the oesophagus and trachea if inhaled. Sorry to say this, but if the kitten died 3 days afterwards, I'd say that the cause of death would have been the damage the peroxide caused, not the LSD - had it been LSD, it would have caused a fatality within the first 12 hours.
If you want to get a cat to vomit, just put something very bitter like lemon juice in its mouth, they generally vomit at the drop of a hat (I've stood in enough piles of kitty puke to say that with total conviction)