A passive HDMI to VGA adapter usually only works if the display hardware driving the HDMI port supports switching to VGA analog signaling -- which is unlikely for MacBook. You could try one of the following:
1) A Mini Displayport to VGA adapter -- these are cheap and many of them work just fine on Macbook. The ones from Apple work really well, and YMMV with cheaper adapters depending on how finicky the projector/display is that's being connected. Your Thunderbolt port also serves as a Mini Displayport -- no need to buy Thunderbolt specific hardware.
2) An active HDMI to VGA adapter. This would do the downconversion to VGA externally. It would be an extra box and probably more expensive than #1 above (even from Apple).
3) A USB 2.0 to VGA DisplayLink type external video card. This might not work all that well if the EinScan-S cameras become confused by the brief macroblocking effect that you can see whenever the screen changes.