Personally, I don’t have a problem with mosquitos biting me, and if you do, you can gently move them away from you by brushing them with your hand. Spiders I usually just let outside if they are in my house and leave them alone when they are outside. With dog fleas, I am guilty of killing them and feel bad but don’t have an especially great way of getting rid of them.
I don’t have an issue with killing a mosquito, tick, or flea that is going to bite my dog or me. Veganism is all about reducing your harm to animals as much as possible, but don’t let the fact that you’d like to be able to kill mosquitos stop you from going vegan and saving many other chickens pigs cows etc.
I wear mozzie bands in the summer so I don’t get bitten and I don’t kill spiders. I do have a cat which I de flea, ie by putting drops on his neck. But what is the alternative? I cannot let my cat suffer from a flea infestation which would be detrimental to his health.