I recently bought the Synth V upgrade and upgraded all of my voices but haven't had time to do much with it yet. My general approach in the past has been,

1) write basic lyrics and basic melody
2) enter chords into BIAB, generate some tracks and export to WAVs
3) generate additional tracks from EZ or NI
4) play and record additional instrument tracks
5) record my own vocals and decide if I like them for the song
6) if I do like my vox I'll continue to refine everything in my DAW (Reaper)
7) if I don't like my vocals or I need additional parts for harmonies or duets that is when I bring in Synth V
8) when I use Synth V I try to start by having it convert my vocals to MIDI but fixing the results is sometimes more tedious than just putting in the notes directly!

On my last song I tried bringing Synth V into the process early on. As soon as I got a rough backing track done I entered notes for the melody and lyrics into Synth V to develop the song before investing in all the other areas. For the final version I replaced the Synth V vocals with my own but they still helped me write the song.

Oh and, on the subject of using these tools together, I NEVER do that! I find every one of these tools has a heaping helping of quirks and bugs and they all work differently! For me it is a lot easier to use the tools individually and deal with all that one tool at a time rather than chase bugs between multiple programs when loaded together!