Is it safe to put vinegar in your bath while pregnant to relieve itching?

Of course Mild Acid. Vinegar is a mild/weak solution of acetic acid. Acid suppresses the growth of a variety of fungus ; bacteria. Several species of bacteria commonly growing in vaginal secretions (called lactobacilli - for Lactic Acid production) produce acids which suppress fungal growth. Antibiotics killing off these bacteria commonly leaves the territory open for fugal/yeast growth with resulting problems.