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