I am so embarrassed because I really should remember this but I don't. What I do remember is that for some reason Spike and Buffy end up in a "you and me against the world scenario" (they are vampire and slayer, this is not an AH) maybe because of a spell. Maybe because of amnesia. I don't remember exactly why but while under the influence they end up hiding away on a beach in a town north (?) of Sunnydale and while they are there they have a little ceremony, just the two of them, where they say wedding vows. The spell (or amnesia) breaks while they are there and broken hearted Buffy leaves Spike and runs home.

Later, possibly after Spike follows her to Sunnydale to try to get her back and finally gives up Spike goes back to their little beach cottage to lick is wounds and a short time later Buffy starts dating Riley to try to get over her Spuffy feelings and as a result of a coincidence they end having their picnic lunch date on "their" beach and Spike sees which sets him off on an anger spree. I don't think he has the chip. I don't remember if he never got it or if it stopped working, though.

Help?

The more I think about it, the more I think that it was during the Something Blue engagement spell and they may have been on their way to Vegas and then were too impatient so they simply decided to say their own vows on the nearby beach that had a deserted cabin on it.

It's a huge fic. Epic length. IIRC it's over 200,000 words.

I think you are after Gem of a Soul by spuffy luvr. Pretty sure all of that happens in part 2 (chap 17) of that.

OMG Yes! Thank you!