Dumbledore and Snape's Unbreakable Vow?

[Najwa]

No: HPFGUIDX 151210

Najwa:
Dumbledore trusted Snape for some odd reason. I never understood it,
until I recently reread the Unbreakable Vow. It's not dark magic, so
why wouldn't Dumbledore make this Vow with Snape when he let him work
for him? Dumbledore is no Narcissa Malfoy and would undoubtedly word
it very precisely, and at the same time he has no selfish intentions.
He probably would have said things along the line of do whatever is
necessary to protect Harry Potter and the Order of the Phoenix and
make sure that Voldemort will not win; or something to that nature.
Maybe Dumbledore telepathically pleaded with Snape to kill him,
because it would be of some benefit to Harry and perhaps others. I
highly doubt that it was never used before and I highly doubt JKR
would let us in on this Vow without it being important not only to the
book but to the series. What do you all think? Sorry if this might
have been brought up before by the way.