I would never douche. I've not heard anything about soaking a tampon, so not sure there, but douching is really not healthy for your vag.
Can't say I'd put lime juice in there either.

If you are looking for ways to make your secretions more acidic and sperm friendly, look into the diets and food you should and shouldn't eat to change the chemical makeup that way. Putting lime juice in your vag may give you more than you bargined for!