According to the release notes for the latest Vista WHQL driver
(181.20) for your card, the resolution 1920 x 1080 is not mentioned at all. It does, however, mention 1280 x 720, but only at 16-bit and 8-bit colour depths.
That does not mean to say that your resolution isn't supported - it is, as you have found out, but only at a low frequency.
Going back to driver version 163.75, the resolution 1280 x 720 is not mentioned.
It would be a fair assumption that, since 1280 x 720 is now partially supported, support for 1920 x 1080 could be included in a forthcoming driver release.