Indeed, while UA's normal policy is to provide domestic vouchers, many people have gotten int'l vouchers in exchange. Search for "voucher exchange" and/or "voucher international exchange" and you'll find the threads - there are plenty of them.

Basically, just explain that you earned it on an int'l flight and you fly mostly internationally... they should have no problem exchanging it in that case. This is especially true if you live internationally. (Vouchers earned on domestic flights might be more problematic to exchange, but then again, maybe not.)