Hi smak,
Your and samborsu's posts induced me to do some research on the Chinese internet and in my books. As a result I believe now that it is Ming dynasty Zhangzhou export ware. Zhangzhou ware is what formerly was erroneously called Swatow ware; but it is not a kiln per se, but rather the export location of multiple kilns, that gave the wares the name. The two larger kilns supplying Zhangzhou wares were Zhangpu and Heping kilns. Both are mentioned in various sources (also shipwreck porcelain) with plates/chargers of a similar design. I did not find an exactly similar decoration, but many plates have similar floral designs, without the outer rim decoration. I found that rimon other Zhangzhou plates, though.
I even found an item in a book that said it was Yuan, but I doubt this is correct. There are too many instances related to the above mentioned Ming ceramics. The base of the plate also supports late Ming.
(Peripheral information:
Zhangzhou wares were exported in large numbers to Japan and the west until the last Ming sea ban was hindering export (although it is said that they continued smuggling the porcelain out by ship even then). Afterwards Zhangzhou decreased in importance and the main export port shifted to Quanzhou, farther north, as far as I know.)
My personal view is that despite the above explanation does not preclude one hundred percent that the item could be Vietnamese. Somehow I feel that I saw this decoration on Vietnamese items too. But this would come as no surprise as the last Ming sea ban is thought to have been one of the reasons that kiln workers emigrated to Vietnam (no work), which in turn would be the reason for many similar blue and white decorations there. Maybe someone with sufficient books or documentation on Vietnamese shipwreck porcelain could check this...even if it is only to remove that possibility.
