What you're using is not a converter but an adapter, big difference. First DVI is a digital signal and VGA is an analogue signal. That adapter only works when the DVI port supports both signals (called a DVI-I port as mentioned above. The DVI port on your card is DVI-D which only supports/carries a digital signal so it won't work with that adapter as there is no analogue signal. For that you need something like this, Newegg.com - StarTech High Resolution Video DVI to VGA Converter DVI2VGACON DVI to VGA Interface
to convert the digital DVI signal to analogue for VGA. You can read more about the various types of DVI here, Digital Visual Interface - Wikipedia, the free encyclopedia
As far as those Samsung monitors only having a VGA port on them when the majority of video cards no longer have a native VGA port and in most cases only support VGA from one of two DVI ports on the card. Here is someone with the same problem of running two VGA only monitors on a card with two DVI ports, EAH HD6870 multiple display problem
Not sure why Samsung is selling monitors that only have VGA inputs unless it is due to a lot of laptops that are out today have a VGA port, and to be used as a second or bigger screen for them (a lot of laptops also have a HDMI port as well, but you can only use either the HDMI port or the VGA port at one time).