If you spray a DIY vinegar solution that contains high salt, it can disrupt the soil's structure and make it difficult for plants to grow. That's why it's vital to avoid spraying vinegar solution on areas where you are trying to grow plants and flowers. When using a mix of vinegar and soap, some say you can plant a few days after the vinegar dries. 

Yes, weeds will grow through pea gravel but you can prevent this during the installation process. Install a weed barrier underneath pea gravel and swap it out every three years to keep weeds at bay.

No, it's not really possible to completely prevent weed growth. Weeds are incredibly resilient—all they usually need is a tiny bit of dirt to sprout, so their eventual appearance is pretty much inevitable. However, you can definitely keep them at bay by getting rid of weeds as soon as you notice them. The longer you let them sit and grow roots, the more they'll spread.

The main problem with crabgrass is that it produces thousands of seeds that can remain dormant until the following season. To get rid of it, you have to remove existing plants before they have a chance to spread seeds and stop old seeds from germinating.