However, bringing a gift no matter how small is always a good thing I think no matter what custom.

it may not be appropriate. Nothing that could be considered celebratory or brightly-colored or "against the mourning" would be. No major life decisions can be made for 3 years and celebrations like weddings or parties must be delayed until the end of mourning. (Not for you, for the immediate family. You can get married or buy flowers. They're not supposed to do anything not in-line with the mourning-period ritual.)

That's apparently the old-guard super-traditional. Newer less-formal traditions are increasingly common where less and less of that is done beyond the feasts and religious ceremonies of the mourning period.

I think the best course is to ask. Courtesy and respect towards custom you're unclear on is always appreciated.

Here's where I was reading about it all. Funeral rituals and customs are interesting to me from a social and anthropological standpoint; this was one of the more involved I've read about.

http://www.vanhoaviet.info/mourning.htm

"In Vietnam, the family of the deceased undergo a ritual after 100 days of them passing away, were the whole family sits in pairs in a long line up to a single member of the family. A monk (Thay Cung) will place a thin piece of cotton over the family member's head and ring a bell and chant while rotating the bell around the deceased's head, sending them in to a trance and open a way for the deceased to return to the living. A bamboo tree with only leaves on the top with small pices of paper with the deceased's name written on them will start to wave when the deceased is coming. They believe that after 100 days the deceased may return to this realm and "possess" the body of the member of the family undergoing the ritual and once it is completed the other members of the family can communicate with the spirit of the deceased through the tranced family member.
Normally this ritual will take all day to prepare and then as long as 6 hours praying and chanting, changing the family member at the front of the line. Afterwards they will then burn a paper house and paper made possessions (that which the deceased would have loved during his life) so that they may take it trough to their next life with them."

You've probably found this yourself. It does not specify whether it's appropriate to bring a give, but from my tour in '68 and my limited contact with the population, I would think it would not be unwelcome. These ceremonies are always designed to ease the grief of family members and a gift can only facilitate that.

Flowers are pretty universal to "rebirth" ceremonies and wine helps dull pain, so...

I think you should follow your instincts and your DIL will appreciate the gesture, graciously.

The Vietnamese are a very gracious culture.

I just spoke to a Vietnamese gentleman that our company had done business with a few months ago. He says that flowers or a fruit basket would be an appropriate gift to mark the occasion. He said that they often give birds, but I don't think we're going to go for that.

If you are handing something to a person older or otherwise deserving special respect, always use both hands.