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.

As I recall the original Nintendo (NES) came with a 9V AC PSU.
Due to the design of that system, it's indifferent to AC or DC, so it can run on a DC supply but the original PSU that came with the console was AC.
Unfortunately I think the original AC PSUs are kind of pricey though because of demand.
There's probably a lot of cheaper modern PSUs marketed on eBay for that application but they might put out DC instead of the AC that you require.

I had a US Robotics modem that also used 9V AC. I found somebody with that PSU here:
https://www.ebay.com/itm/322716870021 (not mine, no affiliation, etc, and there might be cheaper ones if you look more)
It's rated at 1A. I don't know if that's enough for the VIC-20 (somebody else can comment).
There's a lot more of those modem PSUs on eBay but it's a little irritating that USR didn't put the ratings on the obvious, outward facing label, so most listings don't show that information and some of them could be higher voltages than 9VAC.

Edit:
I just saw your first post! Thanks! Sorry, had to scroll up. 😁

Thanks! You just reminded me that I should have an extra Nintendo Power Supply, so I dug it out, and it's a Virtual Boy DC10V adapter! Blah, wasn't expecting that 😀

I do have a NES power supply I can try out though, and I also have a C64 1764 Power Supply -- hopefully it works. Looking at the C64 PSU it says that it outputs 9 VAC 1.0A along with 5 VDC. Not sure if I can tap the 9V pin and use it?

Edit, deleted this question, since your first response answered it.

MCGA wrote on 2021-04-02, 02:21:

Looking at the C64 PSU it says that it outputs 9 VAC 1.0A along with 5 VDC. Not sure if I can tap the 9V pin and use it?

As long as you make sure to get the pins right then that ought to work. I'm guessing that's probably a common mod among people who have both systems, so there might be some posts/tutorials/videos/whatever about doing that with the C64 supply.

Luckily the prior owner of this C64 PSU drew the pin layout on the side. But I'm going to pickup a digital multi meter to make sure it's right. I'm not good at reading my analogue one. :]

I made a working PSU, and the best part is that after much research I found out that DC works on the AC models and it runs cooler.
This forum was super helpful:
http://sleepingelephant.com/ipw-web/bulletin/ … it=Power+Supply

I'm using a 9V DC 3A adapter that was probably for a notebook. I soldered it to a square 2 prong plug I found at the same pawn shop. 😀

I also made a video cable for my VIC 20, but since I'm using a 5 DIN Keyboard to PS2 adapter, there's no audio( the reset pin isn't connected through. ), but it at least let me see if the VIC 2o was working. Going to order the 5 DIN connector and make a proper cable with video and audio later this month.