Fausto,

Although I have no relation with Microsoft, I wil give you my 2 cents as a Microsoft Certification "Junkie". What I'm telling here is no rule from Microsoft, just the way I look at it from experience.

An exam is retired if one of the following conditions is true.
A. An exam about an application is retired when an exam about a new version is released that is 2 version higher. (VFP3 exam was retired when the exam about VFP6 was released, with VFP5 in between)
B. An exam about an application is retired when an exam about a new version that is 1 version higher was released 1 year ago. The NT4 exams and the VB5 exams are a good example about this.
C. No new version is planned (either by lack of demand or lack of a new version of the program) and the old one is too ancient.

For you situation:
When the rules that I saw the last years aren't changed, the VFP6 exam's are retired when:
A. 1 Years after the VFP7 exams are released
B. When the VFP8 exams are released.
C. Let's hope this one never happend

I think that A will never happen (too little differences between the products to create a new exam) and that B will take some time.
And like Garret mentioned, when (enough?) VFP developers don't use the opertunity to get VFP certified, Microsoft will think twice if a certification (path) with a new version of VFP is still valid.

So my conclusion would be:
Take the VFP6 exam(s) when you're ready for it. (The certfox book from www.hentzenwerke.com is the book to use for it, there is no book that explains the thing you need to learn to pass the vfp6 exams.)
Not just because the VFP6 will not retire soon, but also to give Microsoft the message that Microsoft should provide the resources to create an exam for a new version of VFP. (Personally, I think this would be for VFP8, not VFP7)

Regards,