If it's a steroid medicine, such as Budesonide, then the formal requirement is a GP letter or copy of the prescription (print out of the e-prescription is OK too so long as it has your name on it, which they may reconcile with your passport). Experienced HAL staff know most medicines and probably would only glance at the prescription. There is a bit on HAL's website that says it should be checked in to hold luggage except for enough for the trip - luckily it's just the website, HAL's staff are laid back on that one and in any case with foam you can't split up that medicine yourself. Doha is laid back, they are ok with most in transit medicines and they aren't funny about UK prescribed medicines.

Keep it in a separate 1 litre bag from the normal 100ml liquids bag, mixing it in is not a good idea.

I'm not sure of the situation with medicines in Vietnam, I expect it's fine, but if it is steroid then you really must not stop the course of treatment midway, so I would see the point in putting one canister in hold luggage, another perhaps half used in hand luggage, just in case of some accident or damage along the way.