The 6510 instruction set was identical to that of the 6502, I believe. More importantly, the core I/O routines in the C64's ROM - the so-called "kernal" [sic] - were similar to those in the VIC-20 (I think they even had same entry points) so the "port" was pretty trivial.

The BBS code was originally largely devoted to menu generation, the bulletin and message databases, and of course the routine that drove the modem and console simultaneously (including conversion of Commodore's weird not-really-ASCII so that upper and lower case didn't appear inverted), with provisions for a keystroke to interrupt a menu or message while it was printing (a feature I was very proud of as, believe it or not, many of the prevalent Apple/TRS-80/etc. BBS's of the time didn't do that - remember that at 300 baud printing an entire screen of text could take many seconds!) None of that code had to change from the VIC to the C64.

It also helped that things like file management were largely handled by the 154x disk drive itself (it had its own 6502 and operating system!) so that didn't need to change either. Of course the additional memory allowed me to add a lot of new features to the C64 version going forward.

With respect to your question about encouragement, I have to say that my parents - my father in particular - were initially quite ambivalent about my sudden obsession with computers (which followed on the heels of obsessions with electronics and, earlier, chemistry). My dad had, it transpired later, read some sensationalized newspaper article about the hackers at MIT and how they spent all their time programming, ignoring their studies and so forth, and I guess he didn't want that to happen to me.

That said, he always, always came through in the end. He'd bought me the massive chemistry set I'd begged for when I was a little kid; he would take me down to Canal Street to browse the surplus store for electronic parts; he'd bought me my first oscilloscope out of my "savings account" that I'd probably depleted ten times over; and ultimately, as I said, he did buy me that VIC-20 and peripherals. (He also had a hard-and-fast rule: any book, on any subject, that I wanted he'd buy for me.)

And later, when I was in high school and getting deeply into Lisp (and math) and expressing interest in (what was then called) AI, he cold-called Marvin frickin' Minsky whom he'd read about in the papers ("I've got this kid..." - my dad was just... like... that), and next thing we knew we flew to Boston where we spent an incredible morning at Minsky's house talking about connectionist vs. symbolic approaches, followed by an unforgettable tour by one of his grad students of the AI lab. Yeah, the exact same den of iniquity my dad had originally worried I'd end up in :-)

So, yeah. I guess you could say he encouraged me, in his own way, even if he worried about me a lot. (It's also true that, whatever I was doing on my own, I always did well in school, so that probably assuaged some of his early fears.)

I don't think my parents quite realized what I was up to, once I got involved in the BBS world. My initial excuse for asking for the VICmodem was actually that I'd be able to call up my school's PDP-11/45 (which I certainly did end up doing a lot) but of course BBS's were very alluring and I spent many a late night clicking away while the family slept. I ran into quite a few shady characters "online" - fortunately as a street-smart NYC kid I think I navigated this pretty well, but in retrospect (especially as a parent myself now) I'm kind of shocked at how much independence I had at 12 or 13, facilitated of course by the significant money I was making from the BBS business. My parents just looked on, bemused, and didn't get involved at all other than to go with me to the bank to help me open a checking account.

Anyway, it was a good childhood.