Stefan, it's not so much the hypernova of stars like VY CMa that could affect life on Earth but the gamma ray burst (GRB) associated with it. The general consensus is that any GRB within the Milky Way galaxy could have potentially devastating effects on life on Earth. But they have to be aimed directly at Earth to have an impact (most of the energy is focused in two directly-opposed 'jets'). GRBs are rare beasts and we've only ever observed them in distant galaxies. There have been calcs done that estimate the Milky Way rate may be between one every 100,000 years and one every million years. Add to that the rarity of one being aimed directly at Earth and you can breathe a little easier.
But only for now! It's possible that GRBs have been involved in mass extinctions on Earth in the past. Who knows, with GRBs travelling at very, very close to the speed of light and with virtually no warning from optical brightening, we could all be toast in the morning!
Check it out on Wikipedia or similar site, just make sure it's referenced.
Cheers -
