I suspect you can check your bag through. If so zero problems. You should check with CX.

If you can not check through and assuming you are a U.S. citizen, you will need to clear immigration and will therefore need a VN visa. Two options.. Visa on Arrival (VOA) or visa in advance from an overseas VN Embassy/Counsel. With a VOA, 1:35 is tight. I'd get a visa in advance in which case, 1:35 is fine. 20~30 minutes max to clear immigration, pick up bags and clear customs. Two minutes to get from arrivals to departure. Check in, immigration and security on the REP flight should take no more than 10 or 15 minutes (there is a priority line for J in security). I fly internationally out of SGN 2 or 3 times a month in J and never check in more than 60 minutes in advance.