There are both VGA-HDMI and HDMI-VGA easily powered from a spare USB port, sometimes even HDMI bus power. I’ve heard the data can be converted line-by-line and

There are both VGA-HDMI and HDMI-VGA easily powered from a spare USB port, sometimes even HDMI bus power. I’ve heard the data can be converted line-by-line and timings are close enough that such devices never need to hold the complete framebuffer.