Q:

Genesis 1:6 — can you clarify this vav and the ensuing phrase?

The sixth verse of Genesis goes,

וַיֹּאמֶר אֱלֹהִים יְהִי רָקִיעַ בְּתוֹךְ הַמָּיִם וִיהִי מַבְדִּיל בֵּין מַיִם לָמָיִם
And God said: 'Let there be a firmament in the midst of the waters, and let it divide the waters from the waters.'

I have a question about the final clause, after the etnachta:

וִיהִי מַבְדִּיל, בֵּין מַיִם לָמָיִם
And let it divide the waters from the waters.

How would you parse the word וִיהִי (veehee)? I am a little thrown off by the vav, because it is not a vav consecutive yet I do not often see vav conjunctives used with verbs. Is it in fact a vav conjunctive, and can these be used with verbs just like nouns?
It seems to be the case, but I just  wanted to make sure, and clarify the construction.
Secondly, how do you translate בֵּין מַיִם לָמָיִם? A word-by-word translation would give something like, "among waters to the waters". But this is not good English. Is there some useful idiom I can learn from this phrase, to arrive at a proper translation, such as "the waters from the waters"?

A:

The vav in ויהי at the beginning of the second half of Genesis 1:6 is the same as the vav at the beginning of the verse in ויאמר, namely the vav of inversion. This use of vav changes a future tense verb to past tense. See https://en.wikipedia.org/wiki/Waw_%28letter%29 .
Regarding the diacritics of ויהי, there is no patach shva hirik because the word ויהי is a combined verb with מבדיל and therefore the pronunciation needs to be shortened to combine the two words, similar to "smichut" of two nouns in Hebrew or a contraction in English.
The meaning of מבדיל בין מים למים is to separate between fluid, unformed entities; the oceans of the earth and cosmic oceans. The translation is idiomatic, as in Deuteronomy 17:8 (MT): 

כִּי יִפָּלֵא מִמְּךָ דָבָר לַמִּשְׁפָּט, בֵּין-דָּם לְדָם בֵּין-דִּין לְדִין וּבֵין נֶגַע לָנֶגַע
  דִּבְרֵי רִיבֹת, בִּשְׁעָרֶיךָ וְקַמְתָּ וְעָלִיתָ אֶל-הַמָּקוֹם, אֲשֶׁר יִבְחַר יְהוָה אֱלֹהֶיךָ בּוֹ

where the same construction is used to indicate differentiation between similar entities that might otherwise be confused.

