What is vegur appearing in HTTP headers?

It's a Heroku proxy/load-balancer adding the Via header.

---

See: Vegur at GitHub.

Heroku's proxy library based on a forked Cowboy frontend (Cowboyku). This library handles proxying in Heroku's routing stack.

The via header shows proxies and their names. In this case, the client went through a proxy called "vegur" - either its software or the computer name.