I've heard other stories about people having problems with Cubase and latency, and I suspect it might have something to do with the monitoring situation (ie. you don't hear a delay while recording because Cubase automatically delays the playback to compensate for the hardware), but the recorded track is recorded later in time (relatively) than when it was performed. Is this the case or is it just latency in the monitoring during the recording/overdubbing stage?

Make sure you have the latest drivers. Check the buffer size settings in Cubase and set them smaller if you want lower latency. Clicks may occur if you try pushing it too far.

If the Tascam drivers are crap or don't work with Vista, then download a version of Asio4all which is a generic asio driver.

Asio4All is good but you can't get the best latency with it, the drivers for your sound card will be the best.

I don't know what are computer specs, but Vista is being a real resource hog, you might want to try and revert to XP, it will make things run faster.