  It is possible that measurements of vector boson scattering (VBS) at the LHC
will reveal disagreement with Standard Model predictions, but no new particles
will be observed directly. The task is then to learn as much as possible about
the new physics from a VBS analysis carried within the framework of the
Effective Field Theory (EFT). In this paper we discuss issues related to the
correct usage of the EFT when the WW invariant mass is not directly accessible
experimentally, as in purely leptonic W decay channels. The strategies for
future data analyses in case such scenario indeed occurs are proposed.
