That DVD would like to start selling WL converted rooms before the cabins are done doesn't need slow sales at Poly to generate such a desire. Regardless of Poly, Disney would want to do it if it could. If Poly were actually selling very well rather than the other way around, then DVD would definitely want to speed up the process at VWL to assure a selling resort was available in case Poly sold out. I doubt the rumor has any real validity as is, but if someone simply said DVD wants to get the converted units at WL done as quickly as possible so it can start selling those without any reference to PVB that would not be a surprise.
Mentioned above is the belief that Florida law requires some certain proportion of the resort to be done before DVD can sell points. That is incorrect because there is no such law. Legally, a developer could start sellng timeshare units even before it breaks ground for construction. However, there are legal requirements that result, as a practicable matter, in DVD's avoiding selling until construction is well along. Those legal requirements include: (a) you cannot sell until you have filed the detailed offering documents which need to contain accurate descriptions of what the resort is going to be when finished and what the point structure is going to be; (b) you cannot sell with an initial use year that could possibly end before the purchaser could reasonably stay at the resort; (c) buyers have a right to rescind and get any money back if the resort as finished does not comport with the offering docments or does not open on or before the promised time of opening, and, until the resort is complete and open, the develoiper is subject to either an escrow of sales proceeds or other financial guarantee requirements that would cover the rescission of the sales. Because of such restrictions, you probably won't see see sales until well under a year before at least a substantial phase of the resort is open.
Thus far, BLT is the resort that had the earliest pre-opening sales, starting about 10 months before it opened while offering a first use year that began 6 months before it opened. I doubt you will see VWL starting sales that early. AKV was a dual opening resort, first for Jambo, and then not quite a year later for Kidani. However, when Jambo was sold, DVD provided what was going to occur at Kidani, including the number and kind of rooms and the point structure for the entire resort. DVD could start selling converted WL rooms before the beach cabins but it could not do so without telling purchasers what the cabins are going to be and their point structure.