Both nano and (most implementations of) vi have an optional line wrapping feature, where they replace the previous space by a line break¹ when the line becomes longer than a certain length. They behave differently when the cursor isn't at the end of the line, but it's fundamentally the same feature.
To disable this feature in nano, press Alt+L. In nvi, set
wrapmagin to 0 (
:set wl=0 wm=0).
Vim also has a number of more advanced wrapping features which nano doesn't have an equivalent for.
If a line is too wide for the terminal, vi shows it wrapped by the terminal. Nano doesn't, you need to scroll left and right. If that's what you meant by “act like vi”, you're out of luck.
¹ Not a CR/LF. CR/LF is the encoding of line breaks on Windows. On Unix, a line break is encoded as LF.