I'm responsible for several conference rooms and have setup 1080p projectors and I provide both HDMI and VGA connectivity. HDMI for DisplayPort and Mini-DisplayPort, and VGA as a fallback, universal option. Contrary to what I expected, people seem to have much more trouble with the HDMI than VGA, so VGA gets used a lot more than you'd think (even as most workstation laptops made in the last 3-4 years have DisplayPort or Mini-DisplayPort...). Also to my surprise, VGA outputs over 1080p on a 50ft cable run with very minimal degradation on certain laptops - other laptops just don't offer 1080p as a resolution choice and top out at 1600x1200 or something else. Specific example: a ThinkPad W530 will do 1080p, a W520 won't, over VGA. (both do 1080p over displayport/mini-DP)

What determines what resolutions a laptop is willing to output over VGA?

I'm thinking this will come down to either a video driver that says it supports only certain resolutions for output, or limitations of the RAMDAC (which wouldn't be in play, at least DAC wise, on a digital output, but WOULD on VGA, an analog output).

The basic reason for the question is that I noticed, say, a ThinkPad W520 with 1080p built in display, will output 1080p fine over DisplayPort to a 1080p projector, but will cap out at 1600x1200 (practically the same pixel count, just a little shy) on VGA. Now, this wouldn't be surprising at all except SOME laptops have no issue outputting 1080p over VGA, even with lower native resolutions.

Why do I care? Well if there's some way I could enable it... for situations where my users end up using VGA anyway, it's preferable for display mirroring if they can output their laptop's native resolution, which, you guessed it, is very often 1080p on 15" models.

DISCLAIMER: This is primarily a curiosity, I'm not claiming 1080p over VGA is ideal by any means, but hey, if it works. I've seen HDMI start artifacting more over same-length, same gauge cabling (up to 50' run in certain rooms). If you think this is better suited to SuperUser, please move it, but this is framed from an IT standpoint of something that affects a real pool of users in a multiple conference room, 50+ deployed laptop scenario.