The latest version of the VCL specification / documentation can be found here: VCL: ValueSet Compose Language - Guidance for FHIR IG Creation v0.1.0
There is also a simple initial web app for exploring the use of VCL here: VCL Viewer - Value Set Composition Language
The design and development of VCL was inspired by SNOMED’s Expression Constraint Language (ECL) but is grounded in the semantics of FHIR CodeSystems because any VCL expression can be transformed into a corresponding FHIR ValueSet (R6).