Are you wanting to keep your audio effects for vocals separate from your guitar effects? (Helix processes all guitar stuff and guitar looping, and VL3 for vocal stuff and vocal looping?) If so, all you need is to take your raw, unprocessed guitar signal from the Helix into the VL3 so it can detect your chords (with no effects or distortion which would confuse it). You can use a send block from the Helix, before amps and effects, out a send in the Helix into the guitar input on the VL3, and adjust levels properly.
Use a separate output(s) from your Helix and VL3 into your mixer, so you have guitar and vocal channel(s).
If you need to loop both guitar and vocals in synch, things get more interesting. Your loops have to be synched via MIDI clock or you'll have an audio mess. The VL3 has quantized clock synch for loops, the Helix does not. So VL3 would need to be the master, and Helix the slave. Use MIDI out from VL3 to MIDI in on Helix, and read the manuals to figure out how to synch the two via MIDI clock data. Not for the timid, but you should be able to do it.
Another choice to avoid the complexity of operating two synched loopers (and tap dancing) would be to simply take guitar input from Helix and loop it in the VL3 along with your vocals. Or take vocal input from VL3 and loop it in the Helix along with your guitar. I think the looper in the VL3 is better so use that one.
[UPDATE: I just reread your question, and sounds like you want to loop both guitar and vocal in the VL3. Good choice! If so, you will probably want to have two outputs from Helix into VL3; one with unprocessed guitar for the chord detection, and the other processed with effects for the loops. For unprocessed, use a Send as mentioned. For processed, you could use a send, and XLR out, or a 1/4" out (if you're using mono guitar signals to amp and VL3). Lots of ways to get the signal, but keep it simple.
Wow, lots of fun with the setup you have!