We stayed last year in a 3 Bedroom Residence. Our room was not ready when we checked in and so we spent half a day in a standard room. I have not seen the exact room types that you are considering but the room styles are very similar and I studied this in detail when I was in your shoes before our trip.

I believe you'll be happy with the Regency Suite if it's just the two of you. There's no reason to go for the Villa in my opinion. The regular beach is so secluded and not crowded that the Villa private beach is overkill.

I think it is better to stay in the blocks near the main pool area (close to Green House restaurant) for convenience. We stayed in the residences on the North and it was a little far. It's not a terrible walk by any means but in humid Vietnam weather, sometimes you just want to get inside ASAP. Also, the golf carts that you can call for a ride do not always show up quickly.

We tried to upgrade our 3BR Residence paid stay into a Villa using my DSUs and emailing the GM. No dice. I think it's a longshot to upgrade the Regency Suite into a Villa, but also unnecessary. You'll overachieve if you get the Executive Suite upgrade for free, but again I think that is also unnecessary.

You will have a good stay here. It is one of the nicer Hyatts. Everything is cheap in Vietnam. The food at the pool house restaurant for lunch was very good and inexpensive. We also got room service a few nights and it was quite reasonable and the service was great. Even the hotel spa was bearable in USD, and we never ordinarily pay for massages at hotels.