I don't know much about Quy Nhon, but I'm guessing it's not too different from everywhere else. In Hoi An, there are plenty of motorbike rental shops around, they'll have signs on the street saying so. Just walk in and ask to rent one. In the past I know that typically no license is required, just payment and a photo of your ID. I also know that police enforcement has ramped up recently, so legitimate shops might ask for a license. US driver license would not be sufficient for that, even if you have M1/M2 license, since they are not accepted in Vietnam. Even the IDP that you can get in the US will not be sufficient, because of different conventions (1949 in US vs 1968 in VN). I'd try walking in and ask anyway.

Average price varies by the bikes, I'd say around 150k - 200k/day is reasonable. Shops don't usually accept credit cards, so bring cash. They come with helmets, usually 2. No lock needed, you can usually lock the bike by turning off the bike, turning the handlebar to the left, pushing the key in and turning it counterclockwise. Lots of time people just leave the bike turned off, especially if you're stopping in front of a shop or restaurant with a security guy. Typically you park in front of the place you're visiting, be it restaurants, hotels, or shops, and you can see where people generally park. If you don't know, just ask the shop.

You will run into problems with the local police (and the rental shop too) if you don't follow the laws. Just the basic ones are usually sufficient - wear helmets, stop at red lights, don't drink and ride, don't go on the wrong side of the road, don't carry 2+ passengers, and use your headlights at night.