This is an interesting bird. The extensive head streaking does suggest Vega, but you'll notice it's noticeably grey in tone, rather than brown. While not at all diagnostic, it doesn't fit many Vega Gulls.
A more critical feature, however, is the progression of primary moult. It can be seen that (numbering outwards) P6, P7 and P8 are full grown, but P9 is not visible. Primary moult is, thus, still ongoing. Given the early March date, this means that this bird is not pure Vega Gull, or at least not an individual of the type seen in large numbers in Japan.
Do you have photographs that show the leg colour or upperwing pattern? It would be interesting to see these.
GeoffC
