Vinyl is made of plastic so it might become brittle after a couple years. It depends on what climate you live in. The strong point of aluminum fence is that it's maintenance free so you don't have to paint it every so often like steel or iron fence. It really depends on the look you want around your yard or house. If you have a pool, you will want to get aluminum pool fence to make local codes. I think there are a few vinyl styles that should make pool code as well.

Key considerations imo is what you are calling "oxidizing" and if it is indeed aluminum. Aluminum naturally form's aluminum oxide to protect it self from moisture and polutants. I doubt that happening except at joints of painted surface. In any event wax and silicone are the last things I would put on outdoor furniture simply because it negate's possibility of painting in the future. Assuming it actually is aluminum,my first action would be powerwash,coin carwash or scrub with stiff brush and soapy water. If the white haze remain's only at joints and scratches,soaking and gentle scrub with white vinegar on plastic dish scrub/super fine steel wool should remove most of it. If the haze remain's on most surface I'd apply good outdoor enamal paint.

I have not used charcoal grey or brown stains. Blues and greens do not hold up outside. Have used black and red and it was on concrete that had been surface ground with a carborundum stone grinder. The black was dark and harsh like a fresh hair dye job at first, but then faded over a year's time to a nice taupe grey, and I like it just fine. (I applied it twice and let it set overnight the 2nd time.) The red was orange rust colored and I did not care for it at all. The stains, being watery, spatter easily and stain any masonry object nearby if any gets on it. I don't know how it would stain other materials. I applied it with a broom and got fine splatter mist on nearby masonry. (It does not look horrible but I would have preferred it not happen. Slightly worse, I had areas that ran, bled and pooled into seams between pavers ... and those look like crying after an already bad mascara job.) Something that would mop it on with would probably be better. Mask off anything nearby on which you don't want stain.

I would love to make this into the quintessential outdoor entertainment space. I wouldn't change what is there except remover the fence between the pool and patio. I see a great outdoor kitchen and a bar, both with counter or bar height seating. A few round tables with 4 chairs each around them - must be comfortable!. Add some paddle fans, a misting system. All seating must be comfortable. Around the pool I would have a couple of seating areas with 2 chairs and a small table between them so people who are swimming can sit and dry off or have a drink, also nice at night when it has cooled down and you just want to sit and enjoy the evening. On the far side of the pool I would create a fire pit and seating (far from the house and shed so you don't burn anything down). You could do a horseshoe pit, putting green or ping pong table. Don't forget an outdoor sound system. Think about how you will entertain, who you will have at your home and how you will enjoy using the space. No one cares how modern it looks when they are having fun.