Go there and change. If you worry about changing at the goldsmith shop, you can try those money changer with bank. I usually change mine at Eximbank money changer at the right corner along Dong Khoi street with one of the small lane.
dong khoi is around japanese area ...
there have few stores that u can change ur currencies ...
was thinking why not change in singapore
save the trouble going there to search for which store gives the best rate ...
u wont enjoy the process ...
holiday is to relax and enjoy, not to hunt for shop just to change ur currency ...