At a guess - as long as you can present the voucher to the airline's own agent - it will be valid.
If you were planning a return trip and contacted the carrier by phone and then posted the voucher? (by recorded delivery!) The voucher would probably state that it was only redeemable against a similar sector for which it was issued i.e. a domestic sector.
If the carrier compels you to book the international sectors with them, before accepting the domestic voucher - then that would have to be in the
small print of the agreement that you sign at the time you accept the compensation.
This is a guess, as I have always been unsuccessful in VDB
