If you don't buy a Vignette you will need to take care which roads you use.
A Vignette is not just required for the Autobahn, it is also required for an increasing number of other main roads (generally marked at their entrance with the appropriate sign).
Even recent maps don't mark all these roads - it appears to me that if significant road improvement is undertaken, the road is often re-classified.
In heading to Vienna last year we encountered one minor road in the process of being upgraded, which required a vignette for a very long stretch.
Our van is badged outside at 3500kg, but is in fact uprated to 3850kg. We 'escaped' last year on a vignette, but it is an increasingly risky process. As noted, the go box is an expensive option, and inconvenient.
Austria is one of my favourite destinations, but is now off-limits until/unless the the policy changes.