We propose an integrated vector modulator based on two compact and high-speed germanium-on-silicon Franz–Keldysh electro-absorption modulators. The proposed vector modulator is extremely compact with a total footprint of only 1800 μm × 200 μm. We further experimentally demonstrate a 4-quadrature-amplitude-modulation (4-QAM) at 40 Gb/s over a 20-km standard single-mode fiber transmission. The complex signal is successfully reconstructed with a single-ended photodiode in a recently proposed Kramers–Kronig receiver for future low-cost, low-power, and low-footprint datacenter interconnect applications. The preliminary performance of the vector modulator with a 16-QAM is also investigated.