I dont own that card, but I do have Vista 64 and a Prelude.
I use WASAPI in Foobar.
You will need to set Vista's sound properties to match your source file encoding (i.e. 2 channel, 24bit, 96kHz).
You may need to also set your sound card properties for the SPDIF out to match 2 channel, 24bit, 96kHz. Prelude's have Audio Creation mode with bit perfect playback as an option, plus you have to set digital out settings to 24bit, 96kHz I believe (I use analog currently until I get a DAC and/or an amp).
And finally, you will need to set Foobar's preferences to 24bit and WASAPI output.
Make sure your DAC supports the bit depth, rate, etc.
Of course, make sure you have the latest OS patches, sound card driver, WASAPI plugin version, Foobar version, etc. (this is obvious to most though, but thought I would mention just in case)