After many tries over about 25 days, I was successful in completing the connection between Tiller > Yodlee > Vanguard. But, the real story is about Tiller Customer Support.
The Vanguard conversion process was frustrating but, in my opinion, Tiller support was very good considering the dependency on a 3rd party, Yodlee. Over the course of 25 days, they were proactive in getting back to me several times to see if I was still unable to connect.
Each time, they included detailed steps to fix the problem. Ultimately they escalated the issue to arrange a screen-sharing call with a tech-rep from Yodlee. The day before the call, by again following the same steps prescribed by Tiller that I had tried so many times before, I was finally able to make the conversion.
I’ve been a Tiller customer since 2021. I think their Customer Support is one of the best features of their product.