At Elysian Fields there are these two stories, not exactly what you're looking for with Angel etc, but rather good all the same:
1.
Maybe This is Heaven by effectionSummary: What if Buffy hadn't left Spike to die and close the Hellmouth by himself? What if she stayed with him... what would have happened?
2.
Yes, I Do by Pet Summary: Set at the end of 'Chosen'. What if Buffy had acted differently?
At Fanfiction . net this story, which fits your query, is there, however it's not complete and hasn't been updated since 2005, there could be another site that holds a completed version but I'm not sure:
1.
Forever by 3988AkashaSummary: Post S7. Buffy never left Spike alone in the Hellmouth. What happens when Angel and crew get the amulet? What has changed Buffy and Spike, and why are they back?
And at Bloodshedverse they have the following story which involves Spike and Buffy remaining in the Hellmouth and then being transported somewhere else (but not through the amulet). It's also a WIP (but has been updated this year):
1.
Holiday on Helarth by dawnofmeSummary: Buffy refuses to leave Spike to die in the high school basement. When she takes the amulet off, they end up in another dimension. Alive, wounded and exhausted, they must deal with a world that's very different from their own, yet eerily familiar. It's not Hawaii or a cruise in the Caribbean, but they manage to make a holiday of it.
I'm sorry that most of these don't fit your request, but I hope you enjoy reading them.
Sarah