Moving from Arc to Vivaldi

Vivaldi's built-in importer treats Arc like any Chromium browser: per a Vivaldi team member, it can bring passwords, history and extensions, but not Spaces or sidebar folders, because Arc doesn't store them as bookmarks (Vivaldi forum). It can import a bookmarks HTML file, so that's the route.

Steps

Back up first. Copy StorableSidebar.json somewhere safe (where it lives).
Run Arc's Export Data. Mac: Arc > Settings > Advanced > Export Data. Windows: Ctrl+, then Advanced > Export Spaces and tabs > Export Data. You get Arc Data Export.md, a list of links per Space.
Turn the sidebar into a bookmarks HTML file with an open-source converter such as arc-export (unofficial; read it before running), or with the kit's script.

In Vivaldi: File > Import from Applications or Files, run the Arc/Chromium import for passwords and history, then import the bookmarks HTML file.
Create one Workspace per Arc Space and open that Space's folder in it. Use Tab Stacks for Arc folders.

Arc vs Vivaldi, briefly

Arc	Vivaldi
Spaces	Workspaces
Folders	Tab Stacks / bookmark folders
Split View	Tab Tiling
Favorites	Speed Dial or pinned tabs
Profiles	Profiles

Vivaldi also runs on Linux, Android and iOS, which Arc didn't. The trade-off is setup time: it's very configurable. Sources: Workspaces, tab management. Full table on compare.

FAQ

Vivaldi says the Arc import succeeded but my Spaces are missing. Why?

Arc Spaces and sidebar folders aren't bookmarks, so Vivaldi's importer can't see them. Convert them to a bookmarks HTML file and import that.

Does Vivaldi have something like Arc Spaces?

Yes, Workspaces. They don't swipe like Arc Spaces, but they group tabs the same way.

Related: compare alternatives · quiz · export guide · all guides

Want it done in one sitting?

Everything above is free. If you'd rather follow one checklist with tested scripts (Mac and Windows) and a verification step, the Arc Migration Kit is $12 with a 30-day refund. You don't need it if the free route works for you.