Poloch was spot on.
I live in HCMC and go to Vung Tau for the weekend.
As a Platinum you get more out of your stay at the Pullman. Cheaper rooms, Executive lounge (only open in the evening for two hours) and free drinks at the Lobby bar.
However if you are a regular you notice the little hickups. Mainly since the new management beginning of the year, they are more revenue driven and the perks for returning guests get smaller.

The Mercure is only 4 Stars and you can notice the difference but what really speaks for the Mercure is the location and the views.
If I would go for the first time and spend a weekend I would try both hotels.

At least they are not disapointing.