Individual letter pairs that don't begin any roots - vav

Looking first at vav. It occurs as the second letter of a root with every letter in the alphabet. So all its one-sided non-uses are in the first position of a root va-vd, vz-v't, vc-vm, vs, vy, vx-vr, vt - 16 of the 37 empty single slots in the table of the 401 concordance pages - see this post.

vav and its pattern of usage in the first two letters of a Hebrew root

 There are lots and lots of examples of v followed by other letters. They are pervasive. Vav is a connector of words used for and but so or if - then and many other variations.

But very few roots begin with vav. There is only one that is itself Hebrew, and that is the word vav itself. Just as vav (also written as waw) connects words together, so also it is the hook that connects all the pieces of the tabernacle together. 

All 28 of the words beginning with vav in the Hebrew Bible can be scanned in the smallest volume of pages in the concordance beginning here. 

vwti		vwti	ושתי Vashti Esther 1:9
vv		vvi	ווי the hooks of Exodus 27:10
vhb		-vhb	והב Vehav Numbers 21:14
vpsi		-vopsi	ופסי Vopsi Numbers 13:14
vwni		vwni	ושני Vashni 1 Chronicles 6:13
vnih		vnih	וניה Vanyah Ezra 10:36

Note the importance of vav in this visual Midrash as outlined in this post by David Z. Moster, "Scribing the Tabernacle: A Visual Midrash Embedded in the Torah Scroll" TheTorah.com (2016). https://thetorah.com/article/scribing-the-tabernacle-a-visual-midrash-embedded-in-the-torah-scroll