The most reliable and secure way to get a Vista Qcow2 image is to download a standard ISO and convert it using QEMU tools. Source the ISO
Instead of looking for a pre-made QCOW2 download, the standard industry practice is to download the official and convert it or install it into a QCOW2 container yourself. This ensures you have a clean, unadulterated copy of the operating system. You will still need a valid product key to activate it, though Vista keys can often be found inexpensively on second-hand software markets or may already be in your possession. Windows Vista Qcow2 Download
This creates a 40 GB sparse file. Actual disk usage grows as data is written. The most reliable and secure way to get
Using QEMU on Linux/macOS/Windows (with QEMU installed): You will still need a valid product key
qemu-img convert -f vmdk input_image.vmdk -O qcow2 output_image.qcow2