Comment by youngtaff

ESI = Edge Side Includes think Server Side Includes on a CDN technology as supported by Akamai and used by sites like Ikea to deliver a fast maintainable experience

VCL = Varnish Configuration Language i.e. how you configure your Fastly services

If you're just using a CDN as a proxy then there's no lock in but plenty of sites are using CDNs for much more than that