Married Male, age 60, bought son's circa 1880's home so he could move out of his in-laws basement after three years. He took a new job in another city and after three years was unable to sell this old, tired home. We sold our hobby farm in order to help him out. Hardwood floors, sixty plus years old. Badly warped. We thought having heat in the home would level them but it only helped slightly after one year. Estimate of $3500 to sand two directions, then refinish. After a new roof, doors, windows and siding, we are running low on funds. I want to put a filler material in all the valleys, something like Durhams Water Putty, to make a level surface, then lay 1/4 plywood underlayment over the HW floors, then lay down vinyl plank. Would this work? Is there another material I could use as a filler? The estimate for just laying a new subfloor, minus the vinyl plank was $4500. Any advice would be greatly appreciated. Thank You.

You have done well to get so much done for you home.
Filling the cupped flooring with any filler will make you have less choices in the future. The filler you are considering will make it almost impossible to sand in the future.
If you are low on funds and can afford a payment schedule for a loan, get the loan and have the wood floors sanded and refinished. If you take care of your floors they will last 20-30 years before they need sanding again. Wood floors are an investment. Take advantage of the fact that they are already installed.