IME clematis do much better without competition from other plants' roots. I also find that the vinca is really aggressive and I am in the process of ripping mine out in many places.

I've had clems growing with vinca and I never found it to be a problem :-) As far as root competition is concerned, I would be more at issue with trees or deeper rooted plants when attempting to get clems established than I would be with a very surface rooted groundcover. And the vinca will indeed serve to reduce soil moisture evaporation, just as any other mulch or groundcover would. Just water slowly and deeply when you do water.

I have problems with tree root competition (severe problems) but not groundcover or perennial root competition.

Thank you for the replies. I will let the vinca cover and make sure to water well. One less chore on the list to worry about :)

Will the vinca climb and combine with the clematis ?

Not if you keep it cut back. TBH, I have never had an issue with Vinca minor climbing adjacent plantings. Growing through them, yes, but never growing up onto them

Thanks for your reply. I have experience with each one just never used together.