San Diego stores this week will be the first in the nation to market a new type of milk, richer in calcium, with "today's fit, health-conscious woman" in mind. Vital 15 was developed by the California Milk Advisory Board and is being introduced with a $1.3-million promotional campaign. It has the same number of calories as low-fat milk but contains more calcium and protein, as well as greater quantities of Vitamins A and C, thiamine, niacin and other nutrients not usually found in such concentrations in milk. The board said development of the product was prompted largely by public concern over osteoporosis, a weakening of the bones that affects one in four elderly women and is thought to be related to insufficient calcium. Marketing of Vital 15 in other parts of California is expected to follow within six months to a year, the board said.