Firstly, I suggest that before using any power supply on valuable hardware, always check it with a multimeter to confirm it's outputting what you expect. In the case of DC supplies that includes checking polarity.
There's a couple potential risks with that PSU.
1) The VIC-20 calls for 9V AC but your PSU is DC. With some devices that's okay but it could be a problem, depending how the VIC-20 is designed internally. In the case of Commodore hardware I think the AC is required for it to function. I don't know if it would also cause damage. Somebody who knows the hardware would have to answer that.
2) Point 1 makes this moot, but just for additional info - in the case of DC supplies like you've proposed, many of them are unregulated. Even though it says 9V on it, it may output a varying voltage depending on the load. That's typical with older, heavy brick type PSUs. If you check the voltage with a meter you might find it's putting out 11V or more at a light load, which might be harmful if you plug it into a system that hasn't been designed for that tolerance.
This is a dangerously misleading thing about the labeling of old power bricks, and a good reason to always check them on a meter.
Modern switching power supplies (which are usually a lot lighter) are regulated to stay at a constant voltage. However they can also have a lot of ripple (high frequency voltage fluctuations) especially if they're cheaply made.
When I'm at thrift stores I like to buy PSUs that I perceive to be high quality and well regulated (first-party laptop PSUs for example). Then it's just a matter of changing the plug to use it on something else that wants the same voltage.
Anyway switching supplies are for DC, not AC, so not an option for you.
The good news is that AC->AC adapters are simple and probably very safe, but I think you do need to find one with the correct 9VAC output rather than attempting to use a DC supply which might ruin your day.