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Erm… written permission from the copyright holder of the original content.
But seriously, what you need is a VHS player and a DVD recorder. There are some companies that make units that have both a VHS player and DVD recorder bundled in one unit. Most of these have built in copyright protection that will not allow you to copy commercial pre-recorded VHS tapes. I have one and have used it to copy over all of those home movies from VHS to DVD.