Crossroads demons

Based on what we saw in earlier seasons it seems like crossroads demons have the power to make simple bargains in exchange for souls. They can, to some extent, negotiate or reject the request of the victim. When Dean was ready to bargain his life for Sam's in All Hell Breaks Loose, the crossroads demon was "authorized" to only offer one year- speeding Dean's trip to Hell. When Sam demanded the demon release Dean from his contract, she explained there was someone higher ranked than her that held his contract and she was not able to cancel the deal.

They reinforced the idea that big deals need to be made by more powerful demons in Abandon All Hope, when Crowley tells the businessman who had been dealing with a female crossroads demon that a bargain as important as his required the king of the crossroads to seal the deal. Even in Somewhere Between Heaven and Hell, the 2 demons tell Crowley that there are a number of deals that need "senior management approval." Granting immortality seems like something that could be part of a crossroads deal, but probably not one made by a run-of-the-mill red eyed demon.

As far as being turned by a vampire- it's an interesting idea. If you exist as a vamp until your time is up, does your soul get dragged to Hell or Purgatory? Does a demon make a new deal with the vampire to ensure the soul is claimed by Hell? Or, as you say, are you spared hell because it voids the deal altogether? I guess if the writers ever decide to go down this path, they have several options available.