awk     '               {for (i=2; i<=NF; i+=2) {                       # every second field is one inside double quotes
                                 gsub (/\|/, "\001", $i)                # replace every delimiter in quoted field by something
                                }
                         FS="|"                                         # set desired field separator
                         $0=$0                                          # rebuild fields
                         for (i=1; i<=NF; i++) {                        # in every new field
                                 gsub ("\001", "|", $i)                 # replace something back to commas
                                }
                        }
                        {for (i=1; i<=NF; i++) print $i}                # print new $i fields
        ' FS="\"" OFS="\"" file
xx
yy
"xyz|zzz"
zzz
12