You can try "vim". It does not have this limit and a lot of other improvements over the classic "vi". You can download Packages for Solaris from sunfreeware.com.
Solaris' vi seems to be limited to lines up to 4095 characters. If only a single line of a file is longer than this limit, vi can't read the file. The readonly option does not help in this case either. If the total size of the file is too big, you get a "Tmp file too large" error.