In my experience that's significantly less common in Vietnam than in Thailand or the Philippines (having been to all three, and dated women in all three).
Of course you see a lot of sexpats, but Vietnamese culture is definitely less sexualized than in other SE Asia countries. It certainly exists literally everywhere, but it is just harder to find outside of the STD infected backpacker areas. The women are pretty adamant about having a bf.
For a long time, it was also more difficult and expensive to get a visa... so the guys looking to just get laid went to the easier countries. That is changing though as Vietnam now has an e-visa.