Your only options are to get the workbooks recreated in v7 or upgrade to v8.
In my opinion I would be talking to the vendor about their mistake. You don't just switch formats without warning your customers and, if customers are unable to make the switch, you make accomodations for them. Though it's nice to switch to the "latest and greatest" it's not always immediately feasable.
Whomever made that decision is immature in their experience. If I didn't get satisfaction from that person I'd be complaining to their manager and higher...maybe even start eye-balling other vendors that know's what they're doing. Very unacceptable.
The following is completely unsupported and likely won't work....but it's worth a try IF your vendor did not use any version 8 features while constructing the workbook. If they did, don't even bother.....
- Make a copy of the workbook
- Open up the copy of the workbook in notepad
- Towards the top of the file, find the "version" element, and change it back to older information:
<workbook version='8.0' xmlns:user='http://www.tableausoftware.com/xml/user'>
<!-- build 7000.13.0312.0939
....Save, cross your fingers, and see if you can get it open in 7...
I sure hope Russell's suggestion works for you. I tested it with a very simple cross-tab and when I opened in version 7 it worked, though it lost the field on the label shelf and the font formatting was totally messed up. Still -- it might give you a starting place.
If the vendor can at least provide screen shots of each view, you could see which fields were placed on which cards / shelves which might make recreating the workbooks more efficient.