Since: Dec 04, 2007
I'm not sure what the "best" way to do it is. But what I would probably try first, is like you said, route the outputs from the interface out to the hardware effects rack, and then output from the effect back into the UX2 inputs. Create the new track to be recorded in the daw and arm it. I'm guessing the signal coming from the effects unit will be in stereo, so you will want to set the new track to record a stereo signal, and set that to the correct inputs from the UX2 (usually paired up as input 1/2 or 3/4, etc.) You'll also want to, disarm the already existing recorded tracks since you just want to record the output from the UX2.
Are the vocals in stereo or mono? (I'm assuming mono) If they're mono, you might be able to pan them far left in the DAW, and just record them from the Left output on the UX2 (saving a cable in the process), run that mono signal into the effects unit, and then run the stereo output from the effects unit back into the UX2 on 2 separate input channels, and record them to a single stereo track in cubase.
If the vocals are multiple tracks with the pannings set up already, then just record the stereo output from the UX2, send those to the effects unit, and record the stereo signal from the effects into a stereo track in the DAW.
Then, mix the wet and dry tracks, maybe adjust how wide the effect track sounds, and you should be good.
I'm not sure if you need to set the existing tracks to be monitored internally in cubase or not. Also, there may be a step or something that I'm forgetting. Maybe set the fader level in the DAW to zero or adjust it so you get a good signal coming from the output (but not too hot), and/or make sure the signal you are recording into the new track is around -12ish DBFS. I know it's basically a loop of sorts, but you probably still don't want to record too hot.
Note: I don't use cubase, I use Reaper, but the process of setting the new track in cubase should be pretty similar since it's a basic function.
Edit: Actually I just realized, you only want to monitor one of the signals, either the playback, or the recorded signal. Due to round trip latency, if you monitor both, there will quite likely be a slight delay between the recording and the playback. And since the speaker outputs on the UX2 will be going into the effect unit, you may have to use the headphone jack on the UX2 to monitor.