Just finished the Vercel coding round last week so sharing while it's fresh. This was for a mid-senior frontend engineer role.
Format. It was NOT a standard OA through HackerRank or Codility. They sent me a GitHub repo and asked me to complete a take-home over 48 hours. The instructions were pretty clear about scope: roughly 3-4 hours of actual work.
The task was building a small feature on top of a Next.js codebase they provided. Specifically something around routing and data fetching. Can't share exact details but it was clearly designed to test real-world skills, not algorithmic puzzles.
Difficulty. Not hard in a LeetCode sense. Hard in a "this codebase has some decisions you might not agree with, figure out how to extend it cleanly" sense. A few things weren't documented. I had to read the existing code to understand patterns before writing anything new.
There was no strict requirement to use specific libraries. I added a dependency for one thing and explained my reasoning in the README I submitted. They seemed fine with that based on my feedback later.
What they cared about. The debrief with the engineering manager touched on: how I navigated ambiguity in the spec, whether my code was readable to someone who didn't write it, and why I made certain architectural choices. Not a single question about time complexity.
They also looked at my git commits. I'd been committing in small logical steps rather than one big final commit and the interviewer mentioned that positively.
Tips. Treat it like real work, not a test. Write decent commit messages. If something is unclear in the prompt, state your assumption in a comment or the README rather than guessing silently. And actually run the tests they include before submitting. Sounds obvious but I imagine a lot of people skip that under time pressure.