Vand isn't a smart man - uneducated but innately clever so to speak, and he sees in Yajarn someone who complements his own weaknesses. Vand is trying to overcome what he sees are incredibly humble beginnings that have made any advancement in life almost impossible.
Yajarn OTOH is adept enough at magery to have been able to undergo the magical transformation into a mojh. In Vand's eyes their relationship is unequal as Vand doesn't feel he can compete magically, the only area he considers of import.
I plan on Vand staying in Mind Witch because he's incapable of the greater magics. So he tries to get the maximal benefit from his less-than-perfect magical abilities by concentrating on combat-related things. Thus he uses the mindblade in lieu of other weapons because at least it's magical. Likewise he views his affiliation with Yajarn in the same way.