The HDMI output to the "Digital Monitor" is 1920x1080 resolution - no problem. The VGA output to the "Monitor" will only allow a resolution of 1600x1200. The monitors are the same.
I just had a laptop and two monitors sent to me for my new job. The laptop is an HP ProBook 450 G1. It is running Windows 7 Professional 64-bit. The monitors are Dell ST2421Ls. The Processor is an Intel Core i7-4702MQ and the graphics card is an Intel HD Graphics 4600.
I have the monitors in Extended mode. The "Digital Monitor" is display 1, and the VGA Monitor is display 2. I came across another forum that discussed the "CustomModeApp". I tried to make changes there (and the settings told me the changes had taken effect - but, in fact, nothing happened). This allowed me to select 1920x1080 (where previously the highest resolution selection was 1600x1200) but there was no change on the monitor - not even after reboot.
I am running the latest driver version from HP. I tried to install the Intel driver, but it was "not validated for this computer".