Why will decreasing my sodium intake help with my meniere's-related vertigo?

It may: Meniere's disease is a chronic, progressive form of dizziness. It usually has 4 symptoms associated with it: 1) hearing loss ( begins as low frequency) 2) dizziness (many times incapacitating) 3) tinnitus 4) pressure in the ear or head. First line treatment is sodium restriction/ diet modification followed by a diuretic. In extreme cases surgery may be indicated.

Fluid overload: Water follows salt. The more salt, or sodium, there is in your tissues, the more fluid you retain. Meniere's disease is a condition of fluid overload in the inner ear to an extreme which we call "hydrops" and it injures the nerves of balance and hearing.

Absolutely: Salt, fat, caffeine, alcohol and tobacco are the enemy when you have menieres disease. The better you watch your diet the better your symptoms will be.