I don't think the panos would necessarily draw anyone to any site. Afterall, they don't even know about them until theay have already arrived. I suppose you could generate a report that can tell you how many visitors arrive on the page containing the pano and how long they stayed.
Another thing you could do is survey customers. Ask them directly what they thought of the VR tour and if it had any influence on their decision to buy. But even that would not be scientific.
I think many people consider VR tours as bells and whistles and many VR companies are selling it that way. "Here's a cool accessory to your website."
I think instead of trying to sell VR tours based on increased sales, you should consider also showing their value based on saving them money. VR tours are a great way to qualify customers before you spend time and money sending a rep to them with a breifcase full of color brochures.
I'm pitching VR tours to an associate of mine in the commercial real estate market. He tells me his clients are impressed with the tours but he's not able to get them to commit to one. I tell him it's because they don't see the value beyond the glitz. I want him to ask them (better yet, I want to be there on his sales calls) about their usual sales process to their clients. How do they get clients? How often do they meet with prospective clients? What do they talk about? What do they show them? Then I want him to talk about how VR tours can save them time and money in the sales process.
I'm not sure how your client sells seats -- maybe to groups? More probably based on shows. But you'll need to find that out in order to customize a sales pitch highlighting the value of your product. If you can't do that, you won't make the sale.