If someone intentionally pours bleach on another individual's grass, it is definitely considered vandalism. Intentionally destroying someone else's property is absolutely destruction, which is considered vandalism under the law.
If the area is large enough to significantly impact the image of your lawn or could impact the sale of your home or affect the price offered then yes it should be considered vandalism. Check with your police department because not every city has the same definition of vandalism.
Any wrong done intentionally to cause damage to someone's property can definitely be classified as vandalism. Pouring bleach intentionally on the grass will definitely not be done to improve the state of the garden.
I'd say it depends. If the grass got damaged then I'd consider it a vandalism. If not, then it is not. There is no vandalism if there is no damage. Vandalism is when willfully DAMAGED somebody else's property. If you did not damaged other people's property then vandalism had not occurred.