I've spent way too much of my free time digging through fics for this ship, and adventure is definitely where it shines. Vivi's world knowledge and political mind combined with Luffy's chaotic, forward momentum creates a dynamic that's perfect for exploration. A lot of post-Alabasta fics just stick them on a random island, but the ones that get it right weave in the larger world.
My absolute top rec would be 'The Desert and The Sea' on AO3. It's a 'what if Vivi stayed' scenario that sends the crew on a detour to a sky island not in canon, but it feels like it could be. The author nails the crew banter, and the adventure is less about a big bad and more about unraveling a mystery tied to an ancient civilization, which plays to both their strengths. The slow-burn aspect is there, but it's woven into the plot so naturally you're just as invested in finding the next clue.
Another gem is 'Crown of the King' on FanFiction.net, though it's a bit older. It's a full rewrite from Reverse Mountain onward with Vivi as a permanent crew member. The adventure arcs are massively expanded, with longer stops at Jaya and Water 7, introducing new threats and allies. It can get a bit overpowered at times, but the sense of scale and discovery is fantastic.
Honestly, filtering by the 'Adventure' and 'World Building' tags on AO3 is your best method; sorting by kudos will surface the big ones, but don't ignore some of the less popular works—I've found a few with incredible puzzle-box plots that just haven't gotten the attention they deserve.