The most important thing is whether you have a visa or a VOA (visa on arrival) letter. If you have an actual visa, 1:35 is easy. If you have a VOA letter, it is still doable but tight.
Typically
- Walk from plane to immigration - 2 minutes (max 5 minutes)
- VOA Letter Processing - 15 to 20 minutes (SKIP THIS IS YOU HAVE AN ACTUAL VISA)
- Immigration - 5 to 10 minutes
- Pickup bags - 15 minutes (after landing)
- Customs - 30 seconds
- Walk to domestic terminal - 3 minutes
- Check in domestic (business class) - 5 minutes
- Clear domestic security (business class) and walk to gate - 5 minutes
It you don't dawdle, you should be at your domestic gate about 30 minutes after getting off your international flight.
On my last trip to SGN, I was plane to curb in under 5 minutes. With a visa (not a VOA) and no bags, the longest it has taken me from plane to curb is a little under 20 minutes.
With a VOA, worst case was 90 minutes, but 90% of the time it is 20 to 30 minutes to the curb.
(I fly into SGN 2 or 3 times a month.)