Can Australians get a visa on arrival in Vietnam when traveling from Thailand?

Vietnam doesn't have visa in arrival anymore. De facto they never had it; there was a visa with the said name but unlike what it suggests, you had to apply for it in advance. They now aligned the name with the reality a few years ago, and made it so that you apply from the government directly instead of being forced to use an agent. Now, Vietnam does have visa exemption, like Thailand. But as an Australian you do not qualify.

So, get a visa in advance is the way to go.