Vietnam isn't a country where it would be easy to carry a pram. You would need to do a fair bit of slinging/carrying. Is your back up to that or would your DH take it on?
If you can't find a tour company in Australia there is a good chance that you could negotiate something in Vietnam.
Most of the more active tours in 'Indochina' assume that you will be able to ride a bike for short distances and walk hence the child thing.
If you can get another family to join you you might be able to negotiate a tailored itinerary.

ETA - Try ringing Intrepid. They have a Vietnam tour that is quite sedate. If there is a bike riding morning you could just elect to go for a walk instead.