and there is no dispute on this matter in masorah. But in popular speech, it often becomes ve'eschanan, with a sheva under the vav rather than a kametz. Thus, a Google search yields the following number of hits:
|כג וָאֶתְחַנַּן, אֶל-ה', בָּעֵת הַהִוא, לֵאמֹר.||23 And I besought the LORD at that time, saying:|
In contrast, with a kametz can also be much more popular:
and so on. I would guess that you can probably find some interesting demographic differences between these two clusters.
Why is it vaetchanan, with a kamatz under the vav? This is the difference between vav hachibbur and vav hahippuch. If it would be vav hachubbur here, there would be a sheva under the vav and the intent would merely be "and", but not to reverse the apparent imperfect (future) tense to perfect (past) tense.
A vav hahipuch, on the other hand, would take etchanan, which is a first person imperfect, and change it into a perfect (past) tense word. And this is what we want, since Moshe is talking about past rather than future actions.
For a typical vav hahipuch, there is a patach under the vav and a dagesh in the next letter. This is not possible here, for the aleph is a guttural letter and thus does not take a dagesh. Instead, we have compensatory lengthening, in which the patach in lengthened into a kamatz. The weirdness of this form, combined with the softness with which we Ashkenazim pronounce the kamatz, is quite likely behind this popular mistaken pronunciation of it as if with a sheva.