Optical VCSEL-based links operating in on-off keying (OOK) modulation represent a robust energy-efficient solution for short-reach optical interconnects in datacenters. We report on the optical and electronic elements of such link and their integration into the transmitter (TR) and receiver (RX) assemblies. A single channel transceiver link capable of 40-56 Gbit/s OOK transmission over multimode fiber at record energy-efficiency of ~4.5 pJ/bit is demonstrated. VCSEL driver and receiver transimpedance amplifier (TIA) circuits capable of generating 80-100 Gbit/s error-free signals are characterized on a special test-board assembly. Real-time 56 Gbit/s transmission experiments of the complete link are done, resulting in bit-error ratios (BER) below standard Forward Error Correction (FEC) levels without equalization or signal processing.