This is the season of Vietnamese wedding, where restaurants are full of wedding parties, so I thought I'd give some thoughts about it for those (un)lucky souls who get invited to a Vietnamese wedding party.
First of all, Vietnamese wedding party is huge compared to the ones you see in the West. It is a family business. An occasion to invite the parents' business contacts, neighbors, and of course, relatives.
It is a chance to show off one's status, especially if you/your parents are official big-shots. The bigger, the better. It got so out of hand so that the government officially puts an upper limit of 600 guests per wedding for government officials and relatives. It also forbids renting 5 star hotels and luxury resorts for the occasion.
Anyway, 300-500 guests are normal. Guests are seated around table of 10 (at least in the South). The party is normally 3-4 hours.
Normally, you'd have diverse "ceremonies": some old traditions like cutting the cake, holding speeches, some new traditions: pouring champagne over a tower of glasses, throwing the bouquet, and of course, singing the karaoke.
Smoking is not required. Tie and dress optional. Smart casual/business casual is normal.
Drinking is always heavy (especially in the rural areas).
Now, one important issue at last: how much to give as a gift? It depends on how much you know the pair or their family, how expensive the wedding. Not the least, your status as a foreigner would automatically result in expectation that you'd give more.
It is not common to give presents as in the West. Money is much more appreciated. I'd say that 1 million VND and upward is what you are expected to give attending a wedding in D1/D3 for a pair from "good" families (middle/upper middle income families). It can go up much more. I was advised to give as much as 500 USD when attending a close friend/relative's wedding.
The locals give 500 000 VND and upward.