The more crowded the tank the more likely neglecting to vacuum will kill your fish. With that being said, we can conclude that not vacuuming can kill fish. Poop settles in the gravel stocking the tank with a permanent (until vacuumed) ammonia / Nitrite farm. The larger the fish the more he poops (Oscars get big and poop very big) The messier the fish the more food waste there is which is also an ammonia / nitrite farm (Oscars are very messy fish)... Concluding... find a way to vacuum every week. (responsibility write off... IMO)
Regarding plants... yes it is difficult to keep plants in an Oscar tank... but totally possible. Just takes extra planning and maintenance. I keep live plants in with 5 inch Oscars and 6 inch Pacus more successfully than I thought possible a year ago. I keep them rooted in plant boxes (no more than 3" wide, length doesn't matter) with A LOT more gravel over them than necessary (5-6"). I also put them in an area that would otherwise be a 'low traffic' area.
My Green Terror loves it as he's the smallest fish in the tank. The Oscars really don't pay the plants any attention at all except when the crickets (feeders) swim to the plants for safety (then I have a mess to clean). But even when the plants do get hit hard the extra gravel keeps them rooted, so just leaves get torn off.
I do not vacuum the plant boxes as the waste is good for the plants and since it's in such a narrow box there is very restricted current carrying the ammonia / nitrites into the water. (This is my non-scientifically researched theory based on logic/experience/education of water currents with little knowledge of the behavioral patterns of ammonia or nitrites)
This method I use by no means removes the problems plants / Oscars have... but it does reduce them to a tolerable level (tolerable to me at least).
Regarding 50% water change without vacuuming -vs.- 10-20% with vacuuming... Not washing the gravel means you have decided to leave the poop in the tank. It's like putting a giant fan in the back yard to blow the stench of dog poop away instead of cleaning it up. Personally I do a 25-33% water change on all my tanks weekly (except the 5 Gal Betta tank, his is done as needed aka monthly). The plant boxes will also make vacuuming much easier.