Steven Gerrard's side were full value for their victory as they scored some excellent goals, with Rafael Camacho, Curtis Jones, Patrik Raitanen and Glen McAuley also on the mark.
The young Reds went in front on 13 minutes when Simmonds netted from close range after a fine move.
Simmonds then made it 2-0 with a header from a corner before Rapid Vienna pulled a goal back.
Summer recruit Raitanen then got in on the act when the young Finn netted his first goal in a red shirt, again from a well worked corner.
Camacho then added another with a superb curled effort before Jones made it 5-2.
Ireland hot-shot McAuley then netted his first goal of pre-season with an excellent finish after superb approach play from Abdi Sharif.