What impedances do graphics cards expect on H-Sync and V-Sync for VGA monitors?
According to the VESA Plug and Display standard, which uses compatible electrical signaling for analog video, the host is supposed to detect a minimally-capable monitor by looking for 75 Ohm termination on the video lines. Anything beyond basic functionality is determined by reading EDID information over the I2C lines. 

Because EDID is so old, the video card may not even bother checking for a monitor without it. You could probably fool the video card into believing a monitor with any desired capability is present with an appropriately programmed I2C EEPROM or microcontroller.