Snape's predicament: The DADA jinx and the Unbreakable Vow

[pansophy2000]

No: HPFGUIDX 136614

Pippin:
"I don't think Dumbledore planned to die that night. But Snape would
know, being the potions master, the signs that someone was in
extremis from poisoning. At the time when Snape arrived on the tower,
he was out of choices. If he tried to save Dumbledore, the vow would
kill him..."



I disagree here...there was nothing about the vow that specified a 
time line for Snape to take over should Draco fall into trouble.  

After all, nothing happened to Snape after Draco's two failed 
attempts to kill DD with the goblet and necklace.  The fact that DD 
never threatened to hurt Draco, IMO, means that Snape was not yet 
obligated by the vow to step in.  Draco could have "tried" to kill 
DD for the rest of his life, and as long as DD didn't try to stop 
him then Snape should be clear...

So why all happened the way it did...I've started to give up trying 
to figure it out...

-pan