Yes, I've been in precisely that situation with precisely those vendors. We would get data in some weird proprietary format that was only readable in the vendor

Yes, I've been in precisely that situation with precisely those vendors. We would get data in some weird proprietary format that was only readable in the vendor's awful desktop application. We ended up spending a couple months reverse engineering these files (they ended up looking like database page tables or something) to finally break out of that ecosystem and get the data in our own databases. It was a lot of work but absolutely worth it in the end. I have a feeling just the ability to natively read and convert those files is worth a lot but it's also the kind of thing that seems like it might be hard to sell...