The LAs are a hard thing to approach, considering the variance among vestiges. I went for the simple +1 for vestiges level 1-4, and +2 for vestiges level 5-8.

I suspect that they would benefit the most from a specific LA based on their individual granted abilities instead of their level, but I'm not that experienced with Binders and Vestiges. For instance, I don't think the abilities Halphax grants are really worth a +3 LA.

If you (or anyone!) have some specific LA suggestions, I'd love to hear them! Though I would like to hear your reasoning behind it too.