There are several money booths at the airport, and the major hotels will change for you too at a lesser rate. I am US so not experienced per se with your currency, but I can't imagine any trouble with it in those places. Maybe in Nha Trang it would be a bit harder, but for SGN I think you're ok.

I agree with the taxi comments, very easy. When you come out of arrivals, just head to the left side as you approach the first lane of the traffic. Each company usually has a rep standing around there, and Vinasun and Mai Linh can be more passive, but walk up to them and state you want a taxi. Mai Linh is easiest to recognize, imo, from the bright green uniform. Anything from 140k to 200k is normal, as traffic has been horrible lately.

Also the museum/cathedral/palace/post office comments I agree with. I do not think the food at Ben Thanh is good at all, but it is certainly a place you can sit and drink and watch the people go by and there are a few gems in the alleys behind it. I think Nguyen Hue is a better recommendation. Enjoy your trip.