Note I haven't booked a full reservation - but I was curious as to how this would work. I put together a short video to demonstrate how to book using Flex Options.
In short - you don't have to book anything with Vistana, you just use your "unit" to select from possible exchanges and the points will be deducted. I think you have to be careful as it doesn't look like Interval knows how may HomeOptions you have, so it might reject the booking if you don't have the options available.
As a side benefit, I noticed that there are no size upgrade fees. Normally 67,100 would just get us a studio at WKORV-N, but it looks like you can book any size without an upgrade fee (I did go right through to the payment section).