Problems booking flight from Ho Chi Minh City to Phu Quoc

Hello,

I'm trying to book a flight from Ho Chi Minh City to Phu Quoc, and the websites I'm using are giving me trouble.

I've tried Vietnam Air and Air Mekong. I was able to find flights and enter my payment information, but at that point, on both websites, when I went to confirm the flight, I got an error message. This isn't a problem with my card; I know that for sure.

Has anyone else run into this problem with these two airlines? If so, how did you book your flights?

We need to book ahead of time because our hotel is requiring that we send them our flight information in order to book a room, so they know for sure we're coming.

I'm located in the United States, so if someone has a U.S. customer service phone number for these two airlines, or a toll-free 1-800 number, I'd greatly appreciate it. I'm trying to avoid calling overseas on my cell phone or else I'll get a huge charge.

Thanks!