Probing the vectorial properties of light–matter interactions inherently requires control over the polarization state of light. The generation of extreme-ultraviolet attosecond pulses has opened new perspectives in measurements of chiral phenomena. However, limited polarization control in this regime prevents the development of advanced vectorial measurement schemes. Here, we establish an extreme-ultraviolet lock-in detection scheme, allowing the isolation and amplification of extremely weak chiral signals, by achieving dynamical polarization control. We demonstrate a time-domain approach to control and modulate the polarization state, and perform its characterization via an in situ measurement. Our approach is based on the collinear superposition of two independent, phase-locked, orthogonally polarized extreme-ultraviolet sources and the control of their relative delay with sub-cycle accuracy. We achieve lock-in detection of magnetic circular dichroism, transferring weak amplitude variations into a phase modulation. This approach holds the potential to significantly extend the scope of vectorial measurements to the attosecond and nanometre frontiers.