Trustee responsibilities fall into three general areas: (i) administration (e.g., bookkeeping and tax filings), (ii) investments, and (iii) distributions (including direct expenditures on behalf of a beneficiary). I'm sure Vanguard's trust company would handle the first two areas well. My concerns are about the third area.
For distribution decisions, I would want to have a trustee which has personnel on the ground locally, and which is committed to the often high-cost, time- and labor-intensive job of investigating a beneficiary's needs, interacting with physicians and caretakers, hiring local service providers, etc. Especially for a special-needs beneficiary. I don't mean this as a knock on Vanguard, but I just would not expect those tasks to be their forte.
In my own will, I have chosen one of the options bsteiner mentions above. After my wife and I are both gone, I've named Vanguard and an individual as co-trustees. I've specified that Vanguard is responsible for general administrative matters and investment decisions (except with respect to assets, like my house, which Vanguard will not manage), while the individual co-trustee is responsible for distribution decisions (and assets Vanguard will not manage). However, I do not have any special needs beneficiaries. If I did, I might choose instead to place all trustee functions (administration, investment, and distribution) with a single local institution with deep expertise in handling special needs issues.