The rule is that all travel must be completed by the voucher expiration date. If you try to make a reservation with a travel date (either outbound or inbound) past the expiration date, Southwest's computer will not let you make the reservation. There is nothing to prevent you from buying two one way tickets: an outbound paid via voucher and an inbound paid with cash. Unlike some airlines, Southwest does not charge more for two one ways than it does for a roundtrip.
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