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The short answer is that there is a flaw in your approach which may prevent you from pursuing one room from the onset:
When Bidding on Priceline the star rating you choose is the minimum star rating you're willing to accept... priceline always has the ability to upgrade you to a higher rated hotel. So even if the Delano is/was the only 4.5* hotel in this zone, your bid for a 4.5 star property could be upgraded to a higher 5 star rated hotel.
The longer answer is that the Delano Las Vegas at Mandalay Bay is currently rated 5 stars by Priceline. In areas where there are 4* and 5* hotels, but no 4.5* hotels... and in these areas it is believed that priceline shows NYOP bidding available at all 3 star ratings. If there is a successful 4.5* bid, it would be upgraded to a 5 star level. I say "it's believed that..." as there is in reality no way to 100% prove that there are no 4.5 star hotels in a given area since it's possible that one has just never been won or reported to the board. However, i have seen countless situations where i am fairly certain there are no 4.5 star hotels, but the zone offers 4*, 4.5*, and 5* bidding.
If the above causes you to exclude bidding for only one room, you may want to consider HOTWIRE or PRICELINE EXPRESS, where you may (with our help) be able to have a high degree of confidence in a hotel that is identified as the Delano and be able to save money by purchasing that way (vs. the cost of bidding for 2 rooms).
What are the date(s) of your stay and what is your budget (both if able to land the Delano and/or if needing 2 rooms)? Also, when discussing your budget, please clarify if that is all-in or 'room only' so we know whether we should be considering resort fees, taxes, etc. in our research.
Please remember to return to BetterBidding and use the PRICELINE and HOTWIRE links on the board to begin your travel purchases... or thru our sister site MAPSandHOTELS.com for your regular rate purchases.
PS- another priceline change you have probably noticed is that it's getting more difficult to find the NYOP bidding page as they continue to move/hide it. We have set up the following site for quick and easy access to the bidding page: BiddingForHotelRooms.com