why aluminium vessels are not washed with alkalies?

Answer 1

Because you can dissolve Aluminium in alkali !
First you dissolve the very thin layer of Al2O3 over the metallic aluminium, it becomes Al(OH)3 that is insoluble in water, but adding more OH- you will have [Al(OH4)]-, and the oxyde will solubilize.
Then Al will oxydize in acqueous alkali, reducing at the same time the water, and giving Al3+ (like before) cation and gaseous H2.
And that is dangerous. Strong exothermic reaction involving H2.