One important piece missing from this article is that on Vercel you do not get global Serverless functions on any plan except the Enterprise plans. By default you can pick one preferred region for your Serverless functions and that's the region that's always used. In practice, assuming you have a somewhat decent caching strategy, this doesn't really matter as far as latency is concerned. Where it could potentially matter is that AWS region having an outage and now you can't fallback to another. We deploy all our functions to at least two regions and Vercel does handle region failover in this case.

Disclaimer: I'm a Vercel enterprise customer