In my experience, this is true. I have expiramented with vacuum tables a little. (Really… just a little, so take this with a grain of salt.) I built a couple of small (8.5" x 11") vacuum tables with both a small 12v vacuum pump from Amazon, and (alternately) using my shop vac. The shop vac version maintains pretty good hold even with a little air leaking once parts are cut through. It does start losing grip as the cut-through surface area expands, but it does a better job than you would think at holding even when a lot of air is leaking.
You can actually hear the tone of the shop vac change as this happens. To see what I mean, just hold your hand over the end of your shop vac to stop the flow of air… the tone will change. Now remove your hand and you’ll hear it go back to normal. The same thing happens as your work piece is cut and more air leaks. If you do this a lot, you get to the point where you can hear when the table is losing holding power and is about to slip.
How much holding power it has vs how much you need depends a lot of the design of what you’re cutting, how thick your depth of cut per pass is, (how much lateral force you put on the work-piece with the end mill), and how much air leaks once it is cut through all the way around your piece. Having a higher friction vacuum table surface helps also. My vacuum tables are HDPE, and they’re very slippery. I think resurfaced MDF might be more slip-resistant.
I originally planned to cut only 8.5x11" work pieces, and so I pocketed a 8.5x11" recess that is .03125 deep for the piece to set down in. That prevents sliding when I am using a blank that fits perfectly, but prevents me from using it for anything larger.
Using a smaller end mill for cut out helps noticably (reducing air leakage). If you’re cutting something thin (<1/4"), using a drag knife can perform the cutout with very little air leak and loss of holding power, but even switching to an 1/8" end mill from a 1/4" end mill has worked for me in some cases. (Cutting paper with a drag knife was my original use-case… this worked mediocre, and I’ve ended up using the vacuum table more for wood than paper.)
It’s worth pointing out that because I only have one shop vac, I have to choose between using my dust collection or my vacuum table.
Bottom-line: for your use-case I think there is a very good chance that a shop-vac powered table will work great. The little $20 12V pump I have doesn’t have the CFM to maintain holding power when the piece is cut through, so if you go that route, look for something with better performance.