How can I retrieve $31K lent to my father-in-law?

My husband and I have maintained an average financial standing throughout our 10-year marriage, diligently saving for our children’s education, which costs about VND50 million monthly. Therefore, in addition to my dowry of VND200 million from my parents, I have also accumulated savings over the years.

My father-in-law asked to borrow money from me four years ago. At 65, he was eager to increase his wealth and managed to persuade my husband, who then convinced me. I agreed to lend the money out of respect for my husband’s family, with the agreement that my father-in-law would pay interest at the prevailing bank rate.

However, the investment has been dormant for the past two years and may never resume. Attempts by my in-laws to contact the investor and recover the funds have been unsuccessful.

Despite this, up until last year, my father-in-law fulfilled his commitment to pay monthly interest using earnings from his two rental properties in Hanoi. But since the start of this year, he has defaulted on these payments and ceased discussing the loan. Whenever I mention the loan, he and my mother-in-law cite health concerns as reasons for their non-payment, leaving me reluctant to press further.

With my children facing a significant exam soon, the need to secure these funds is pressing, yet I find it difficult to insist on repayment.

What should I do?