It was $7000 with a Declaration Card and maybe more now, but some of my friends took more than that and the customs-officers never checked (however, that was another pair of shoes since they could always give the money back to their family-members).
These two websites can confirms what I said:
http://vietnamnet.vn/kinhte/taichinh/2006/02/538840/
http://www8.ttvnol.com/forum/khpl/832161.ttvn?v=fe3s15f489066nsieoom
I just looked at my customs card in my passport, yeah its $7,000 US, with the next box being 15,000,000 Dong (so maybe you can do both - but I don't know how much use that much dong would be elsewhere) and 300 grams of gold, so that could also maybe be an option as well.
