I'm glad this thread helped to uncover the likely originator of this BASIC extension. It is, however, also possible that CBM offered the core of this extension (just the V4 commands) in source code form to licencees, so they could extend it to suit their own needs.
Mike wrote:In the range of $A100 to $A500 I found lots of reference to addresses $935x which seem to act as some I/O registers - quite strange, as on an stock VIC-20 this range ($9200 .. $93FF) points to unconnected memory. [...]
In the meantime I found out, that the VIAs also appear in $9200..$93FF, and those addresses are just other mirrors of VIA #1 ($9110) - presumably used for obfuscation.
There are still those VL-1 or VL-2 cables necessary at the user port to use IEEE functionality, and as IEEE requires quite some extra signals beyond the 8-bit parallel data, those "cables" must contain extra circuitry. Possibly using the 6520 you mentioned.
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BTW, it's either V-Link (of course, the "V" from the VIC-20) or C64-Link. No "C-64 V-Link".