I have been to Vietnam many times now. I found few girls from Vietnam Cupid and kept in touch with them on subsequent trips. I think it could be mix of both where girls are looking for soulmate or opportunity to move to Australia. I must clarify I never promised anyone but was able to be intimate 6 out of 10 girls I met. I am not young anymore but fit and carry myself well. I also a gentleman and treat the girls nicely with nice dinner, bring perfumes for gift. I avoided giving money to girls except one asked to borrow and returned the first time but not the second. So I take it she accepted the money as a gift. My I enjoyed Vietnamese girls as I found them of good values and decent.
I think most of them kept in touch because some of these girls worked hard to make a living and maybe took this as an escape from their routine, go out for dinner and have sex. As I was a foreigner they did not hav e to worry too much or even someone being too clingy or demanding. Most girls mention the local Vietnamese men are rude and disrespectful. It could also the girls are over 30 with kids are less in demand.