I booked a hotel a couple of months ago and was quoted $772 AUD (based on a charge in VND of 14,087,253). The exchange rate worked out at 18247.

This week I was charged the full amount but it was $860 - an 11% increase. The rate works out at 16380 which is just ridiculous.

It seems they can use their "exchange rate may vary" clause to charge whatever they want, with no notice of the significant change and no detail of what rate they have used.

Surely this is not even legal. I won't be using them again.

Hard access to the hotel due to unpaved road.
During rain, the road becomes muddy. Can't travel safe.
The staff recommended that we ride tricycle around, taxi is not available because they refuse to come to the hotel since it is out of nowhere. No street lights at night. We asked for refund, AGODA refused since they have the worst customer service.

They are owned by Priceline, which is a bad company also.

Why should Agoda refund you? Your issue is with the hotel.