Yes. Just be careful not to spray anything on desired plants. White vinegar is good for killing weeds and won't affect the soil near veggies or flowers.
It depends on what you are spraying. Anything that would kill what you have growing and the weeds would be no. It would have to be carefully applied to the leaves of each weed to spare the plants you want to keep, it can't touch the keepers or soak into the soil to affect the roots. If you are spraying fertilizer, yes on green plants and flowers, you can easily foliar feed them. If you want to foliar feed a veggie garden, you will get great green plants and little produce, the roots need to be fed, not the leaves. The only time I foliar feed in the veggie garden is a calcium supplement spray that I use on my tomatoes if by chance I start seeing some blossom end rot. If you are spraying just water, it doesn't hurt as long as it doesn't hurt the plants or flowers. Do not spray them during really sunny time as it can cause the leaves to sunburn and not look very good. It never hurts to dust off the plants.
Anything that kills weeds will also kill or damage other plants.
To focus on individual weeds when spraying, I use cardboard tubes like empty tp rolls. For larger weeds, I plan on using a big oatmeal box with the bottom out. Jewell