What are the themes in Where the Wind Leads? How can I write an essay about one of them?

One theme is family and how this social institution inspires courage, faith, and success.
The author, Dr. Vinh Chung, relates that one of the most influential women in his life was Grandmother Chung, his father's mother. Chung describes his grandmother as a formidable presence. She was almost six feet in height, which meant that she towered over most Asian men and their "diminutive wives." She possessed such "fearless confidence" that she intimidated many who came into her presence.
Grandmother Chung was widowed in 1949, at one of the worst moments in Vietnamese history, when the French and the Japanese had their eyes on the defenseless Asian nation. Dr. Vinh relates that his grandmother had six children to raise on her own after her husband's untimely death at the age of 48.
Because she had little education, the only options open to Grandmother Chung were either servitude or prostitution. Dr. Vinh relates that his grandmother rejected both options and instead founded her own rice milling business. She began by collecting handfuls of rice to mill by hand. The money she made from her first efforts allowed her to purchase more quantities of raw rice to process for sale.
Grandmother Chung built her million dollar rice-milling empire with little more than her resolve and good faith in her efforts. With "no financial resources and little formal schooling," she trained her children to help in the business, and the entire family reaped the results of its industry.
Grandmother Chung's business empire reinforces the importance of family, demonstrating how this social institution inspires faith, courage, and success.
Dr. Vinh notes that his own father, Thanh, soon became the COO (Chief Operating Officer) of the family rice-milling business. Like his mother, Thanh was an exemplary employer; he worked and sweated side by side with the humblest laborer. He knew each employee by name and attended their special family functions. Thanh showed Dr. Vinh the importance of treating employees fairly and humanely. The family's newly established wealth did not prevent Dr. Vinh's elders from treating the poor with compassion.
Hoa Truong was another influential person in Dr. Vinh's life. During his childhood years, Hoa Truong faithfully served Grandmother Chung. The entire family lived in one house. Dr. Vinh relates that his mother's life was made harder by Grandmother Chung's mercurial temper and impossibly high standards.
With the aid of her sister-in-law (who was in the same position as her), Hoa Truong washed the family's laundry by hand, did the food shopping at the market every day, and butchered chickens for dinner. Even after new infants were birthed, Hoa Truong and her sister-in-law were expected to perform the family chores with timely precision. 
Dr. Vinh relates that Hoa Truong would have been considered oppressed by western standards, but by Asian standards, she was an exemplary wife and mother:

If you view my mother through Western eyes, you'll see her as a mistreated individual who should have stood up for her rights; but if you view her the way she saw herself, you'll understand that she made an enormous contribution to an affluent and prosperous family, and to Asian eyes, that is the very definition of success.

To Dr. Vinh, family has always been an inspiration. When the Communists overwhelmed South Vietnam, the entire family braved Malaysian soldiers, Thai pirates, and the tempestuous sea to seek freedom.
Dr. Vinh relates that the family arrived in America with nothing more than the clothes on their backs and that none of the family spoke a word of English. However, the famous Chung industry remained intact. Thanh, Dr. Vinh's father, worked 23 years at an air-conditioning factory before deciding to start a restaurant with his wife, Huo Truong.
The couple's courage, faith, and industry were an inspiration to Dr. Vinh and his siblings. He reports that, today, the Chung family holds "twenty-one university degrees, including five master's and five doctorates from institutions such as Harvard, Yale, Georgetown, Stanford, George Mason, Michigan, and Arkansas." Indeed, the theme of family and all that it inspires is apparent throughout Where the Wind Leads.