At the airport, I'd recommend skipping the fixed rate taxis and instead going up to the departure level to catch a real taxi (Vinasun or Mai Linh). If you just stick with those two, you'll be ok. Should be about $5-$7 depending on traffic and hotel location.
So, here's what happened to us. We paid for an $8 fixed rate ride from "Saigon Air" taxi, figuring it was a reasonable price and less hassle for us Vietnam newbies. But after handing over our receipt and getting in the taxi, the driver says "OK, $15 to hotel". Rather than argue, I told him to go right back around to where we started. He refused, but said "OK OK". Then he demanded payment for the fee to exit the airport (really only 5k and probably included in the fixed rate voucher). I didn't think we should have to pay that either, but I gave him a dollar to shut him up. He said "No, $5". I then told him again to take us back to the airport (we had only made a few hundred meters at this point), but he wouldn't do it. Hard to say whether he understand "airport" and our gestures, though I suspect he did. After 10 rounds of this, he still wouldn't do it. So I said, "OK, pay at hotel." He just kept yelling at us "$5! $5!" and was driving erratically. We were pretty scared at this point, so we just paid up.
I knew we'd have a chance to fix it at the hotel. I jumped out and summarized for the bellboy. He really seemed to understand but then mysteriously walked in the wrong direction. So then I tried the other bellboy and he figured it out more quickly. But by then the driver was already speeding a way and wouldn't stop when the bellboy told him to. I think he got the number, though. He called to the company and they said the driver would return to pay us our $5. Well, that never happened. I spoke with the same staff at the airport two days later but they were unsympathetic. They never gave us our copy of the receipt (my bad for not catching this), which might have been part of the scam.
Anyway, I've since run into other people who fell victim to the same scam from the same company. We later figured out (in part from reading this forum) that if you just stick to the big two companies mentioned above, you won't have any problems. We didn't. Avoid all others (especially sitting taxis or those with tinted windows). Wait if you have to.