Would it be possible to get pics and post theme here?
If it retains all the Japanese markings you can get quite a bit more historical info.
I have used the following website for research in the past.
http://www.radix.net/~bbrown/japanese_markings.html
I know that Japanese rifles were manufactured in China for a time.
It may be possible that production continued after the war and then sold to Viet Nam.
Curved bolts are pretty common with bolt action guns that were adapted to the sniper role.
The Mosin-Nagant M91-30 for example.
Are the serial numbers on the weapon matching?
It may not be a big deal if they aren't as pieces usually get mixed up.
Especially in late war models.
If they do match it could be a purpose built sniper rifle, as opposed to a converted standard issue rifle.