I think emotion and intent are important factors with the magic in the Harry Potter universe. We know they're definitely important for the Unforgivable Curses, from what Bellatrix Lestrange says in Order of the Phoenix:
Never used an Unforgivable Curse before, have you, boy? You need to mean them Potter! You need to really want to cause pain - to enjoy it - righteous anger won't hurt me for long
Splinching when Apparating could easily be related to this - Ron splinches when he's panicked and under pressure trying to escape the Ministry in Deathly Hallows - or to the skill part of magic, which we know is a factor in every spell and is why not everyone is an Animagus or an Occlumens - and why basic magic is something you have to study for seven years at Hogwarts!
If we take emotion, intent, and skill as factors in every spell, then we can extrapolate and say if you were making an Unbreakable Vow like, 'I will revive the dead', both you and the person you were promising it to would have to have a clear idea of what that promise entailed. Are you promising to find the Resurrection Stone? Are you promising to make Inferi? Both of those would be possible to do.
Canon doesn't suggest anything about time limits on Unbreakable Vows - if no time frame was set when you made the Vow, you might be able to make a promise like that and live your whole life, intending to do it at some point. Given you make the promise to another person, though, it's likely that if they don't believe you are holding up your end, the spell begins to take hold.
As we haven't seen spells that involve other people before, it's likely that the second person is needed for more than dramatic tension; it's them you are actually making the promise to, underlined by the fact you need a third person to perform and bind the spell. We could make a logical jump and say the emotions of the person you've made your promise to are the trigger of an Unbreakable Vow.
As an example, if I made a Vow to you that I would never be angry with you, I would live if you didn't perceive that I'd broken my Vow. I could be mad as hell at you and live as long as you didn't know. If you saw that I was angry - if I started yelling and kicking nearby nogtails - I would die. (As well I should. Those poor nogtails.)
I think the Vow we see in Half-Blood Prince backs this view up. Narcissa's words are chosen very pointedly, formally, and carefully when she asks Snape:
will you, to the best of your ability, protect [Draco] from harm?
If we take that line as written in stone, then if Draco stubs his toe when he gets out of bed in the morning (this is assuming Snape's not around, all you naughty fan fic writers), then Snape's off the hook. Draco came to 'harm'; it wasn't within Snape's ability to 'protect him'.
If Snape were to see Draco about to stub his toe and the Vow was word for word literal - boom, dead.
I think we have to assume, then, that it's Narcissa's feelings that matter here more than anything; if she perceives Draco has been harmed and feels it was within Snape's ability to protect him, then Snape's a goner. (Whether a stubbed toe would cut it is a question for another day. She's a pretty protective mother.)
As for what would happen if people tried to make a Vow they weren't completely in agreement on, I don't think it would work.
There's no canon to back this up but we have seen other spells being cast and fail or being cast and only doing a weak version of what they were intended to do, like Harry casting the Cruciatus Curse at Bellatrix Lestrange, and not having it hurt her full-force.
I think we can take that as a mark of how magic works in general, and take it to mean if both parties didn't have a completely clear - and matching - understanding of the promise, the promise wouldn't get made. As the spell is quite a big flashy one with a 'brilliant flame' wrapping around the hands of the people making it with every promise, we can assume there wouldn't be any fire or it would fizzle out, the same way beginners make 'wisps' and indistinct silver shapes before they make a full Patronus. It would be clear that the Vow hadn't worked properly.