Don't worry about onward travel. The form won't ask for it. Do keep your boarding pass upon arrival, though. I've seen someone held because they threw it away.
Vietnam, HCM city was responsive and helpful. I go a non-imm O visa there about a month ago. You should stay for about two weeks to come in on the new visa. I had to rebounce because I only stayed 8 nights. Your visa there is limited by your asserted trip length. BTW, if you go to Phu Quoc, you are visa exempt.