That's simply not true. If your maximum willingness-to-pay for a SaaS solution is, let's say $5 per month, then a Vickrey auction will reveal that and the seller can decide to sell it to you at, let's say, $3.
So you set up a focus group of potential customers and vickrey-auction (or VCG-auction) one subscription (or k subscriptions) among them, for the purpose of recording the bid distribution?
For this to work, wouldn't the participants need to believe that the auction really is the only way they can get a subscription?
Depends on what you would like to focus on. If you want to capture the complete demand curve for a product, then you're right. In that case the participants shouldn't be aware of any other alternative to get the product. But you could also choose to focus on users who aren't willing to pay the current price and better understand their distribution. This would allow you for example to determine which promotion would most likely turn a free user into a paying user.
For this to work, wouldn't the participants need to believe that the auction really is the only way they can get a subscription?