Eh, I think you're underestimating Val's defensive potential.
Shield spell is +4 AC, period. Tower Shield can go up to +9 AC (at +5) +2 Greater Shield Focus, +1/2 Shield Wall teamwork. So that's up to 8/9 AC more. Also her class removes the attack penalty at level 5.
True, for her to tank anything remotely dangerous above Normal, you have to buff the shit out of her (Barkskin, Shield of Faith, Blur, Magic Vestment, maybe Cat's Grace, Protection from Evil) and give her all best defensive stuff (including Dex increasing items, as her class also increases Dex bonus allowance for armor and Tower Shields).
You could go crazy, keep her weapon hand empty and go for the Crane Style defensive feats. That might be going too far though, I think. Particularly that there is supposedly a +3 dodge AC dagger for hardcore defense mode somewhere out there.
So IMO just mix her Tower Shield Specialist with Stalwart Defender, when available. Later on you can dip Vivi or something.
Offense wise, I mainly use her for Dazzling Display aoe shaken.
Of course a custom made, optimized defender build would be considerably better. And her Will save is a pain.