A project I work on uses source code that's been purchased from a third party. The source code is delivered to us in a zip file. We store this source code in version control (TFS 2010) and routinely make modifications to it.
When the third party releases a new version of the product we've purchased, I need to merge their new code into ours. Because the code I get is not under any kind over version control, it's very difficult to do this merge correctly.
The last time I did this, we had not made many changes. As a result, I was able to check all the files out, copy the new files into their place, and fix up everything by hand without the help of any merging tools. Now that we've got a much larger set of changes, fixing things up by hand is too fragile to be a viable option.
Are there any better ideas, or is this doomed to be painful and fragile each time?