I was considering both hotels for a short 2 night stay past November but ended up with staying at Hyatt Regency, where we were upgraded to a large ocean-view suite. Hyatt lounge was excellent and so were the resort facilities. I would love to come back. Note that Hyatt also runs a promo right now.
In Viet nam several Vinpearl properties were recently rebranded to Marriott. On the same trip I stayed in Sheraton Can Tho which still had Vinpearl on their room keys and even AmEx did not recognize the hotel as Marriott property (hence, no bonus points when charged to the branded AmEx). In Can Tho we were not upgraded to a suite - just a room with a river view - and the lounge was a small seating area next to the bar in the hotel lobby. We were also told that the lounge is under a renovation and will be open sometime in the future. Hotel hardware was fine especially when considering the price we paid for the room. I expect something similar in Da Nang.
From what I remember, none of the Marriott properties in Da Nang had lounges before the pandemic. Perhaps, you can call the hotel and simply ask about the lounge if it is important? Note that all food we tried in Da Nang was great and inexpensive.