What the 74VHC157MX is and where it fits
The onsemi 74VHC157MX is a single 4 x 2:1 multiplexer in the 74VHC high-speed CMOS logic family. It selects one of two 4-bit data sources onto a common 4-bit output bus, controlled by a single select line.
No official second-source or pin-compatible alternative is listed from onsemi for this exact order code. The 74VHC family is broadly second-sourced by other logic vendors, but a direct cross-reference would require comparing the full datasheet parametric table.
The 8mA output drive (source and sink) is typical for 74VHC logic — enough to drive a few CMOS inputs or a low-power LED, but not a relay coil or a long backplane trace without a buffer. The 4 x 2:1 configuration gives you four independent 2-input multiplexers in one package. Each multiplexer shares the same select line, so the part is best for routing a 4-bit bus from two sources — for example, selecting between two sensor banks or two memory-mapped peripherals.