unbreakable vows with snape!

[Yuki Lives in Your Closet]

and there was this thing my friend told me and i want to share with u guys:

why isnt snape on the doulble ended unforgivable vow?!
one is the one with narcissa
the other one is with dumbledore?(when he dies the vow breaks?)  

[evil_black_cat]

I don't know.  

[Aquamarine415]

but then what would the one with dumbledore be for?  

[pneoqueen]

yeah snape was stuck between a rock and a hard place there.