You asked this question...here:
Answer given there.
The cheapest way to do this is to fly via KL with AirAsia -- the total in fares should be under US$100. The downside is you'll have to sleep at the airport at the KL-Siem Reap flight leaves at 7am! Not sure about return flights - perhaps the returning flight times are more sensible.
As for VN Air direct, I think you're looking at around US$200 one way.
Yes the Vn offices are open on Saturday -- see their website for details.