My boyfriend and I bought the ticket for the local bus from Savannakhet to Vientiane (75.000 kip) and my boyfriend's wallet (with 400 AUD) was stolen from the backpack that was stored under the bus. Please be aware and keep your personal belongings with yourself.

Hi,
I do not wish to sound unsympathetic here but what on earth was he doing leaving his wallet in that backpack?
If folk need telling to keep belongings of this type on their person then maybe, just maybe, they should not be travelling.
Hope that this does not spoil your travel too much.

As above...its folly , almost anywhere in the world, to leave cash / valuables in an unattended bag.

I endorse #1 & #2 I always have carried high value notes and plastic + passport in a titty bag - surprised OP even dared to mention it.