Its all about the balance...fish-water-food. You can go without vacuuming for years without any negative and possibly positive effect if you don't overstock, don't overfeed(underfeeding an overstocked tank is not a solution) and have good filtration.
Stirring up the mulm inside deeper layers of substrate generally always causes a mini ammonia spike leading to fish problems and algae. So long term you should pick one approach, either vacuum weekly or you only siphon over the surface to pick up the detritus and never disturb the lower layer. It is only natural due to chemical processes that the substrate gets full of unwanted stuff but that's how nitrogen cycle works. The substrate plays a huge role in nitrification....especially if undisturbed for the most part.
From my personal experience I have gone for years without siphoning the substrate in a sand bottom tank...not heavily planted, just some plants. My fish are/were majorly bottom feeders, long barbels always and I've never seen fin rot on any of my fish in this tank. When I took the tank apart after some 4-5 years because it leaked, I was pleasantly surprised of the nice smell of the sand I never ever disturbed.
...But it all depends on the tank set up because in a smaller tank, heavy planted, no siphoned, but overstocked for a few months(not by choice but because the above tank broke) I saw fin rot and fish dying. It wasn't because of the substrate not being cleaned. The tank was simply overstocked and was not functioning healthily biologically. I saw the same thing happening in a third overstocked tank...again not by choice but in this one I did clean the substrate when it was overstocked...same problem....fish issues and deaths. .You'd know if your tank is overstocked if at any one time you see too many fish swimming around
So generally said, don't overstock. Substrate cleaning is irrelevant if the bioload is too much to handle for the tank. And you can skip it forever if the tank is balanced.
