Can you give more information on your connections. You say 2 VGA connections? Wasnt the 5870 equipped with 2 DVI connections, 1 HDMI and 1 Displayport? (My 5850 was configured this way). Are you using VGA adapters since your monitors only have VGA?
While some people have gotten it to work, trying to do EyeFinity over VGA causes issues. For EyeFinity to work best its best to have all digital signals (DVI, HDMI, Displayport). On the 58xx series cards you could only use 2 of the 3 HDMI/DVI connections and the 3rd connection had to be displayport with an active adapter (which you already have). Your issue is most likely in using the VGA on the other 2 adapters. Are you using a dongle to convert the DVI to VGA?