Vietnam in September?

Pleasant, warm days with some light morning or late afternoon showers.

You might look into Mui Ne as a beach honeymoon destination, too.
September is a nice time of year for the VN coast.

I would'nt go to Na Trang for a honeymoon. We were there in mid March this year and its not really that kind of place (outside of the resorts). They are making the town look well by doing up the footpaths and there are a few nice resort places right on the beach but outside of that the twon is not that romantic. They have built a new cable car ride over to the island that you can see from the main beach but we didnt get over there. I've heard that they are trying to make a kind of entertainment island like Sentosa Island off Singapore. There are much nicer places in Muine further south but when we were there you could'nt sunbath on the beach as it was very windy. But it is the place to do kite or wind surfing if thats your thing.

Nha trang is like a horrible spanish resort. Although we had fun

MuiNe or HoiAn are paradise.

Thailand is even better.