When do I need to wash off the vinegar after applying it to my hair?

You can, but it will be hard to get the wax out of your hair, so it might not be a great idea.

I wouldn't recommend it, but if you have to, try to avoid getting soap in the piercing.

Apply it after shampooing the hair.

Yes, you should. Don't forget the after treatment.

You don't have to, but it is much healthier for your face, as it can prevent acne and clogged pores.

It would probably wash out for the most part, but it might leave a tint in your pony's hair.

Use hydrogen peroxide. Need a recipe? Mix together 1/2 cup of baking soda, 1/4 cup of hydrogen peroxide, and 1 tsp dish soap. Apply cleaning mixture onto the grout, wait...

Either will do, they'd have the same effect on the teflon.

If your hair is severely damaged, it will not be possible to get a color that requires pre-lightening, such as pink, without risking severe damage and breakage.

You should do both. Washing your face first gets rid of the excess dirt and oil, making the mask more effective. Washing your face afterwards gets rid of any residue...