cb, I haven't compared the ST and VT panels directly and I don't own either one. Reviews I have read and reports from folks at AVS forum are pretty clear that black levels on the VT30 are improved over the ST30. This may be due to the fact that the ST30 has the Infitie Black 2 Panel and the Infinite Black PRO 2 panel. I think the difference in performance in bright rooms is partly due to the 1 sheet of glass design and also due to the light output from the TV. However, never having compared each one I can't say if this is true or not...
I'm sure the ST30 is a great TV and I was seriously considering getting it before deciding on the VT30. I don't think anyone could go wrong with the ST, GT, or VT model plasmas. We very happy with our lower end TC-P46X3 model panasonic plasma.