Looking at your list:
USB support
Audio in (not XLR)
Preferably HD
Preferably 3CCD
MiniDV or HDD
Most if not all camcorders in this price range will have USB support, but in the HDV or DV models it is only for stills or low quality video; you need to use Firewire to capture. HDD cams will use USB for video transfer.
In this price range you can get SD with 3 CCD (e.g. Panasonic GS500), or HD with 1 CMOS. The new JVC HD7 is coming soon and will have 3 CCDs, but will be well more than £800.
The choices which get closest are:
Sony SR1 (AVCHD HD/HDD): £840 here
http://www.purelygadgets.co.uk/showp...hp?prodid=2796
Sony HC7 (HDV): Just out; £795 here
http://www.purelygadgets.co.uk/showp...hp?prodid=3322
In April the Canon HV20 becomes an option; it is also HDV and a rival to the Sony HC7
http://www.bestcameras.co.uk/shop/ac...amera_338.html
All these models are High Def, have mic input. All are single CMOS. All have USB, but only the SR1 uses it for video transfer.
Vegas can't edit AVCHD yet, but there is a free update to Vegas 7 coming this spring to add it.