Haha, you know where Ngo Dinh Diem murdered ?
You were charged at Chinese restaurant in Saigon ?
Saigon is the only city in Vietnam has Chinatown. So Chinese community is a part of Saigon.
The church you refered is Nha Tho Cha Tam, in Chinatown area which is location for Chinese community ( we call it Cho Lon area). Most of residents there are Chinese ethnic. If you take a look at tourguideline like foodie dot com, tripadvisor ..., maybe you could have better choice. That's why we should study in prior the trip, for saving our money.
Your points are quite exact.
1. Room rates are very good
2. Transport is quite arkward, that's why it's not cheap. Btw, Saigon is quite expensive compare to the remain.
3. Nha Trang is flooded with tourists, is it beautiful? Next, I recommend you to Quy Nhon, or Tuy Hoa ... but a little bit inconvenient because a few resident could speak English.
How much you estimate for accommodation and seafood in Ha Long Bay, 4 stars hotel ?
For me, 4 stars room : my Japanese partner said Excellent : price tag : 80 USD, but to me 38 USD, for tour about 50-60 USD
Fresh seafoods for our big dinner : 8 pax. about 130 USD in total. VIP room. Tip 5$ to waitress but rejected, she get only a half of a dollar only. Choose the right restaurant, nothing more.
Taxi rate now ( mini car ) : 10,000 VND / kilometer = less than 1 dollar for 2 kilometers.
For long range transport, at this moment better choose air over railway.
When your previous trip to Vietnam? Air transport get much attention since then...
As I checked, air ticket in Vietnam cheaper than HSR ticket in China for longer distance. Whileas Vietnam railways is more old fashioned, can't be comparable to even China traditional train.