If it doesn't remove the bacteria itself vacuuming will remove the bacteria's food source. Never understood vacuuming tanks.
The Under Gravel Filter is like any other filter that requires/benefits from some routine cleaning. If not, it can become a plugged up 'nitrate factory'.
What vacuuming does is very basically remove fish poop and debris that could cause a ammonia spike that could affect the livestock.
True as excess detritus can overload a system. However, once decomposition is completed, the resulting sludge is relatively inert material. Assuming routine periodic partial water changes, problems typically only develop with over populated tanks, larger - messy fish, and or over feeding.
You get no anaerobic pockets with under gravel filtration and a gravel base.
True, but there are typically anoxic or anaerobic regions in any deep substrate which is fine - It's only a potential issue if/when organic decomposition occurs anaerobically. This is why I prefer undisturbed sand as nothing organic gets way down under.
I buy bottled water in bulk for my aquariums and for me. Suits us absolutely fine, no algae issues (the tapwater is loaded in phosphates), no chloramine/chlorine, no heavy metals or any other potential nasties.
Many/most bottled water is pumped from deep wells, often filtered, then bottled. Although I've had issues with high nitrates, I appreciate that I have a private well for the aquariums (although we too drink bottled water).
We drink the tap water here, if it were chockfull of chlorine, wouldnt it be harmful to us when drinking it? My water supplier says the concentration in my water is 0,06 mg/l of chlorine, of chloride it is 23,8 mg/l. Their recomendation is to let the water sit "for a minute" to get rid of it.
Municipalities have been using chlorine to ensure water safety for countless years with no objective evidence of any health issues. Frankly, there would be far more health issues if the water was not treated!
Although
@itiwhetu's water (and perhaps yours) may contain low enough levels of chlorine to 'get away' with letting is stand, or spray for a time to allow the chlorine to dissipate, I believe that this would not be the case everywhere and certainly doesn't work if/when chloramine is used.
If you're on a municipal water supply with any hint of chlorine/chloramine, the best and safest approach is to use a a conditioner that dechlorinates the water.
EDIT: @itiwhetu - I worry that your post could all too easily convince a newer hobbyist that he doesn't need a conditioner for chlorine and this could be devastating in regions with greater chlorine or chloramine. A post like yours needs a warning or a disclaimer because although it may appear to work for you, it surely won't work for everyone.