Hey Zen Master. Let me see how best to tackle this. I hope I answer your the actual question you asked :-)

BTW: I'm a HUGE FAN of recording into an external device. I got so sick of latency or just bad sounding recordings through the PC Buss. My TASCAM is my best friend when it comes to recording vocals. Anyway I outlined my steps for you. I'm sure you already do a lot of this.

1: I create my backing track in BIAB (sometimes without accents).
2: Drag into Sonar.
3: Edit the track in Sonar to get the feel that I want.
4: Create individual busses for instrument groupings (like I may have a buss for drums, horns, bass, music, etc..).
5. From Sonar I will create WAV files for each Bus (sometime I'm lazy and will just create a single background music file). My Sonar only accepts wave at the 16 bit setting but that's fine for me.
6: On my DP32 I create the song "Blank" and save.
7: I "Export" the waves from Sonar into the Audio Depot on the DP32
8: "Import the waves into the song in the TASCAM (usually the last 6 or eight tracks depending on the number of busses)
8: Now I can sing into the TASCAM having tracks 1 - 16 to play with. I usually sing the lead on tracks 1 thru 4 and do my harmonies usually starting around track 5 depending on whether I'm doing 3 or 4 part harmony.
9. You are correct that I do all vocals (punch-ins, overdubs, etc..) on the TASCAM. I like the fact that I can "pre-fade" effects (compression, delay, reverb, exciter, etc..) in my TASCAM while I'm singing but yet record the vocals dry.
10: My microphone is a Shure PG-42. Sometimes I will route the mic into a Behringer Modeling pre-amp before going to the TASCAM but not always. Depends on if I'm being lazy.
11: Once I'm done singing I ONLY export the tracks I did vocals on from the Tascam.
12: IT'S AMAZING !! Because I sang against the waves created from SONAR my vocals line up PERFECTLY in the EXPORT/IMPORT!!
13. Import them into SONAR. Drag them onto tracks and start mixing.

I hope I answered your question here. For someone not saavy like you this probably looks like a lot of steps but actually it's not. If I have my tracks already in SONAR it's probably a 10 minute process to create the backing track and move it over. I probably blabbed more than you wanted :-) I would like to do a tutorial on this but I'm not sure how well it would be received on the Band in a Box Forum.

Henry