This should explain the one year thing: "Applicants for Permanent Residence
. . . must apply in your country of residence, your country of nationality or the country where you have been legally admitted for at least one year." What this means is if an applicant was legally admitted for at least one year to a country where they are now working/residing/visiting, that allows them to have their application processed at the visa office that represents that country. So, for example, if someone is visiting (or working in) Egypt and they were admitted to Egypt with authorization to stay for at least one year, they could elect to have their Canadian PR application processed at the Canadian Embassy in Cairo, rather than the embassy that represents their home country.
As far as Vietnam vs Thailand - nationals from both countries are processed through the Canadian Embassy in Singapore. So whether you go to work in Vietnam and your hubby goes there with you or not, it won't affect which Embassy processes his application. If it was two different embassies, he would only be able to have his application processed at the embassy representing Vietnam IF he had been admitted to Vietnam with authorization to remain for at least one year. If he wasn't, his application would have to be processed at the embassy representing Thailand -but because they are the same, it won't matter.
Yes, if there is an interview required, it will be held at the Embassy in Singapore and he would have to go there. It's difficult to know for sure whether an interview will be required - it all depends on the quality of the evidence you submitted to prove that yours is a genuine relationship and not one entered into to facilitate your husband immigrating to Canada. Also, if there are major cultural, educational or age differences, or language barriers between you, you are more likely to be interviewed so that the IO can determine first hand whether s/he believes the relationship to be genuine.