AppleFontSmoothing Has No Effect On My ViewSonic VX2450
A couple things to consider:


Do you see a difference with the various AppleFontSmoothing settings on your main display?
Are you seeing this in all apps, or just one or two? 

Some programs render fonts independently of the OS, so changing the global font smoothing won't make a difference in them

What is the physical connection between the MacBook and the Viewsonic?

mDP to VGA adapter?
mDP to DVI adapter?
mDP to HDMI adapter?

Analog signals on VGA might not support the resolution
Make sure any OSD controls for the port you're using have the correct settings


How have you configured the Viewsonic, relative to your MacBook's main panel?

Mirroring the internal display

Select a resolution for your main display that is an even factor of your Viewsonic's native resolution

Set as its own independent display

Choose the Viewsonic's native resolution




Hope one of these leads you down a path that ends in a pretty picture.