I have my Dreamcast hooked up to my 37-inch LCD via VGA. The resolution it outputs is 720x480 - where I had assumed the Dreamcast was only capable of 640x480? I am using a third-party Naki vga box on my launch (9/9/99) NTSC US model Dreamcast.
Ah yes, I've got one of those Naki produced DC VGA Adapters myself which outputs at 640 x 480 spec. Perhaps your LCD TV monitor is a bit off on the output rate spec. They work like a charm just like the official Sega DC VGA adapter does. ^_~
DC uses a 13.4MHz pixel clock to generate a 640-pixel-wide display on a TV. With VGA output, the pixel clock is doubled to 26.8MHz, BUT... VGA monitors have a quicker horizontal retrace. The normal clock for 640x480x60hz on a VGA monitor is 25.175MHz. So the DC's display will be a little squished horizontally on a VGA monitor, and the monitor may misinterpret it as 720x480.