I had a VIC-20, though solidly in the C64 era; my parents bought it from a hobbyist friend when he upgraded to the C64. I never programmed on it in anything other than BASIC, though (with POKE and PEEK serving the function of inline bits of assembly).
Pedantry: The C64 had a MOS Technology 6510 CPU, a successor to the MOS 6502 used in the VIC-20 and the Apple II. It was designed by people that formerly worked at Motorola, though. And you're also right about the 68k having a flat memory model; it's just that it was a bit later (original Macintosh). The segmented memory model on the Intel architecture bit me in the ass in college, trying to port a simple linked list implementation in C from the school's Suns to my Windows 3.1 machine... the annoying bit being that it was an 80386, which had I been running a better operating system, was perfectly capable of providing a flat view of memory.