If I understood your question, you want to create R a snippet of code, on the fly, and execute it on a R server.
You need to some fixes on your code:
def self.risk_return_plot(stock_ticker = ["VZ", "CHU", "T", "VOD", "DTEGY"])
# creates a R vector version from stock_ticker array
# enclose each item with " char, join with ',' and enclose all with c()
r_vector = "c(#{stock_ticker.map() {|item| "\"#{item}\""}.join(',')})"
# send R command string to the server
RSERVE.eval <<-EOF
myenv <- new.env()
getSymbols(#{r_vector}, env = myenv)
monthly.Return <- do.call(merge, c(eapply(myenv, monthlyReturn), all=FALSE))
names(monthly.Return)[1:ncol(monthly.Return)] <- paste(#{r_vector}, 1:ncol(monthly.Return), sep="")
EOF
end