It certainly helps.. In my planned itineray I had to pass through Phnom Penh anyhow.. So that is going to be a win-win situation
Most PP Guesthouses will get your VN visa for you. They only charge a few USD for this service. It is not worth your time/effort to do it yourself IMO. Fast costs more when obtaining a VN visa. Allow them 1-2 days and you will be charged less. I have used Lucky, Lucky Motorcycle Rentals on Monivong Blvd. many times. They will do it same day, 1-2 days, your choice. Reliable, cheap service. Good luck with your planning.