I have a Dell E207WFP connected to an Early 2015 11" MacBook Air on Yosemite (10.10.5) (then upgraded to El Capitan 10.11.2) via a VGA cable. The monitor is documented to support 1680x1050@60Hz, but the Mac's Displays preference pane only offers 1280x1024 and 800x600. If I option-click on Scaled, it expands to offer 1920x1080, 1400x1050, 1344x1008, 1280x1024, 1280x960, 1280x768, 1280x720, 1024x768, and 800x600, but these still don't include the monitor's actual native resolution. A 15" Retina Early 2013 MacBook Pro running El Capitan (10.11.2) detects the monitor and its native resolution correctly over the same VGA cable.
I tried forcing the monitor to RGB mode using the instructions at https://apple.stackexchange.com/a/106937/43392, which folks on that post said helped Dell U2713H, U2713HM, and U2711 monitors, but it didn't improve the situation for my monitor.
System Information | Displays pane claims the monitor is a "13.9-inch (800 x 600)", which is, of course, incorrect.
How can I convince the MacBook Air to use the monitor's native resolution?