I wanted to do exactly what you want, so I created a script with one native applescript call:
do shell script "curl '" & "http://download.finance.yahoo.com/d/quotes.csv?s=USDCNY=X&f=l1" & "' "
set cell "RMB" of current record to result
I removed the "| vis" because it added some characters.
Make sure the field "RMB" (or whatever you call yours) is on the layout when you call the script.