Based on my research, a Vietnamese citizen also needs a visa to transit Schengen. Given that the OP's gf was denied twice by the US I suspect all EU countries are also going to deny the visa request.

There are 24 countries that a Vietnamese citizen can go without a visa and the closest to PLS that is any significant size is PTY. But I can't find a single routing to PTY that doesn't touch a visa-required country like the US or Canada. If the OP's gf has an APEC business card then HKG opens up as a transfer point not requiring a visa but that, too, still routes through EU/US/Canada.

Sorry, OP. I think your best bet is to ask for the FFC to be converted to the ETC. They should be sympathetic to your pleas and hopefully oblige. It doesn't really hurt UA either way as they keep your money until it's spent. And if you have a serious plan to book a ticket to SGN then go ahead and book it with the same agent on the phone (of course this is not required).

Good luck. Let us know what happens.

-RM